feat(podcasts): concurrent download pool — fair share, tickets, admin tunables

Replaces the sequential worker with a download-manager-style pool
(§0 item 4). Model: the user's connection is the limit; semantics are
fair-share ACROSS feeds, not goroutine-per-book (a global FIFO would
make Book B wait behind Book A's 100 files).

- Atomic claim: GetNextQueuedEpisode + SetPodcastEpisodeDownloading
  was a read-then-write race under concurrency; ClaimQueuedPodcastEpisode
  converts queued→downloading in ONE guarded statement (two claimants,
  one winner — the loser gets ErrNoRows). SetPodcastEpisodeDownloading
  retired.
- Queue tickets: download_queue_pos BIGINT fed by a Postgres SEQUENCE
  stamped at enqueue — the enqueue loop's ordering IS the claim order
  (the foundation §0 item 5 builds on; previously all rows in a batch
  shared one updated_at, so within-feed order was arbitrary).
- Pool: up to podcast_download_concurrency (default 4, live range 1–16)
  claim-loop goroutines per drain round; the setting re-reads every
  round so admin changes take effect without restart.
- Fair share: claim prefers the feed least-recently-served
  (pickFeedToServe; never-served first), earliest ticket within a feed —
  books proceed in parallel while chapters land in order (start
  listening while the rest download).
- Bandwidth: shared token bucket across all workers
  (podcast_download_speed_mbps, default 0 = unlimited); take() reserves
  bandwidth and callers sleep outside the lock.
- Settings: new float setting type end to end (registry defaults +
  clamping, save validation, admin UI number-with-step input) + the two
  schema seeds; worker wires the registry via SetSettings (tests run on
  registered defaults).
- Tests: unit coverage for the bucket reservation math + fair-share
  pick; integration TestPodcastDownloadPool pins ticket order = enqueue
  order, claim exclusivity, and a full 3-episode pool drain.

Dev-DB note: the column/sequence/settings were applied live (idempotent
DDL, same statements the schema runs at next boot) since the running
container predates them.

templates/admin_settings.templ

@@ -244,6 +244,20 @@ templ TunableSettingRow(e SettingEntry) {
 					}
 					class="input py-1.5 text-sm w-32"
 				/>
+			} else if e.Type == "float" {
+				<input
+					type="number"
+					step="any"
+					name="value"
+					value={ e.Value }
+					if e.Min != "" {
+						min={ e.Min }
+					}
+					if e.Max != "" {
+						max={ e.Max }
+					}
+					class="input py-1.5 text-sm w-32"
+				/>
 			} else {
 				<input
 					type="text"

templates/admin_settings_templ.go

@@ -813,26 +813,86 @@ func TunableSettingRow(e SettingEntry) templ.Component {
 			if templ_7745c5c3_Err != nil {
 				return templ_7745c5c3_Err
 			}
-		} else {
-			templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 104, "<input type=\"text\" name=\"value\" value=\"")
+		} else if e.Type == "float" {
+			templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 104, "<input type=\"number\" step=\"any\" name=\"value\" value=\"")
 			if templ_7745c5c3_Err != nil {
 				return templ_7745c5c3_Err
 			}
 			var templ_7745c5c3_Var22 string
 			templ_7745c5c3_Var22, templ_7745c5c3_Err = templ.ResolveAttributeValue(e.Value)
 			if templ_7745c5c3_Err != nil {
-				return templ.Error{Err: templ_7745c5c3_Err, FileName: `templates/admin_settings.templ`, Line: 251, Col: 20}
+				return templ.Error{Err: templ_7745c5c3_Err, FileName: `templates/admin_settings.templ`, Line: 252, Col: 20}
 			}
 			_, templ_7745c5c3_Err = templ_7745c5c3_Buffer.WriteString(templ_7745c5c3_Var22)
 			if templ_7745c5c3_Err != nil {
 				return templ_7745c5c3_Err
 			}
-			templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 105, "\" class=\"input py-1.5 text-sm w-40\"> ")
+			templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 105, "\"")
+			if templ_7745c5c3_Err != nil {
+				return templ_7745c5c3_Err
+			}
+			if e.Min != "" {
+				templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 106, " min=\"")
+				if templ_7745c5c3_Err != nil {
+					return templ_7745c5c3_Err
+				}
+				var templ_7745c5c3_Var23 string
+				templ_7745c5c3_Var23, templ_7745c5c3_Err = templ.ResolveAttributeValue(e.Min)
+				if templ_7745c5c3_Err != nil {
+					return templ.Error{Err: templ_7745c5c3_Err, FileName: `templates/admin_settings.templ`, Line: 254, Col: 17}
+				}
+				_, templ_7745c5c3_Err = templ_7745c5c3_Buffer.WriteString(templ_7745c5c3_Var23)
+				if templ_7745c5c3_Err != nil {
+					return templ_7745c5c3_Err
+				}
+				templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 107, "\"")
+				if templ_7745c5c3_Err != nil {
+					return templ_7745c5c3_Err
+				}
+			}
+			if e.Max != "" {
+				templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 108, " max=\"")
+				if templ_7745c5c3_Err != nil {
+					return templ_7745c5c3_Err
+				}
+				var templ_7745c5c3_Var24 string
+				templ_7745c5c3_Var24, templ_7745c5c3_Err = templ.ResolveAttributeValue(e.Max)
+				if templ_7745c5c3_Err != nil {
+					return templ.Error{Err: templ_7745c5c3_Err, FileName: `templates/admin_settings.templ`, Line: 257, Col: 17}
+				}
+				_, templ_7745c5c3_Err = templ_7745c5c3_Buffer.WriteString(templ_7745c5c3_Var24)
+				if templ_7745c5c3_Err != nil {
+					return templ_7745c5c3_Err
+				}
+				templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 109, "\"")
+				if templ_7745c5c3_Err != nil {
+					return templ_7745c5c3_Err
+				}
+			}
+			templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 110, " class=\"input py-1.5 text-sm w-32\"> ")
+			if templ_7745c5c3_Err != nil {
+				return templ_7745c5c3_Err
+			}
+		} else {
+			templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 111, "<input type=\"text\" name=\"value\" value=\"")
+			if templ_7745c5c3_Err != nil {
+				return templ_7745c5c3_Err
+			}
+			var templ_7745c5c3_Var25 string
+			templ_7745c5c3_Var25, templ_7745c5c3_Err = templ.ResolveAttributeValue(e.Value)
+			if templ_7745c5c3_Err != nil {
+				return templ.Error{Err: templ_7745c5c3_Err, FileName: `templates/admin_settings.templ`, Line: 265, Col: 20}
+			}
+			_, templ_7745c5c3_Err = templ_7745c5c3_Buffer.WriteString(templ_7745c5c3_Var25)
+			if templ_7745c5c3_Err != nil {
+				return templ_7745c5c3_Err
+			}
+			templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 112, "\" class=\"input py-1.5 text-sm w-40\"> ")
 			if templ_7745c5c3_Err != nil {
 				return templ_7745c5c3_Err
 			}
 		}
-		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 106, "<button type=\"submit\" class=\"btn btn-secondary px-3 py-1.5 text-sm\" hx-disabled-elt=\"this\">")
+		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 113, "<button type=\"submit\" class=\"btn btn-secondary px-3 py-1.5 text-sm\" hx-disabled-elt=\"this\">")
 		if templ_7745c5c3_Err != nil {
 			return templ_7745c5c3_Err
 		}
@@ -840,20 +900,20 @@ func TunableSettingRow(e SettingEntry) templ.Component {
 		if templ_7745c5c3_Err != nil {
 			return templ_7745c5c3_Err
 		}
-		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 107, "Save</button></form><span id=\"")
+		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 114, "Save</button></form><span id=\"")
 		if templ_7745c5c3_Err != nil {
 			return templ_7745c5c3_Err
 		}
-		var templ_7745c5c3_Var23 string
-		templ_7745c5c3_Var23, templ_7745c5c3_Err = templ.ResolveAttributeValue("status-" + e.Key)
+		var templ_7745c5c3_Var26 string
+		templ_7745c5c3_Var26, templ_7745c5c3_Err = templ.ResolveAttributeValue("status-" + e.Key)
 		if templ_7745c5c3_Err != nil {
-			return templ.Error{Err: templ_7745c5c3_Err, FileName: `templates/admin_settings.templ`, Line: 260, Col: 30}
+			return templ.Error{Err: templ_7745c5c3_Err, FileName: `templates/admin_settings.templ`, Line: 274, Col: 30}
 		}
-		_, templ_7745c5c3_Err = templ_7745c5c3_Buffer.WriteString(templ_7745c5c3_Var23)
+		_, templ_7745c5c3_Err = templ_7745c5c3_Buffer.WriteString(templ_7745c5c3_Var26)
 		if templ_7745c5c3_Err != nil {
 			return templ_7745c5c3_Err
 		}
-		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 108, "\" class=\"text-xs w-24 text-right\" style=\"color: var(--text-secondary)\"></span></div>")
+		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 115, "\" class=\"text-xs w-24 text-right\" style=\"color: var(--text-secondary)\"></span></div>")
 		if templ_7745c5c3_Err != nil {
 			return templ_7745c5c3_Err
 		}
@@ -880,12 +940,12 @@ func AboutSection() templ.Component {
 			}()
 		}
 		ctx = templ.InitializeContext(ctx)
-		templ_7745c5c3_Var24 := templ.GetChildren(ctx)
-		if templ_7745c5c3_Var24 == nil {
-			templ_7745c5c3_Var24 = templ.NopComponent
+		templ_7745c5c3_Var27 := templ.GetChildren(ctx)
+		if templ_7745c5c3_Var27 == nil {
+			templ_7745c5c3_Var27 = templ.NopComponent
 		}
 		ctx = templ.ClearChildren(ctx)
-		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 109, "<div class=\"mt-6 card p-6\"><div class=\"flex items-center gap-2 mb-4\">")
+		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 116, "<div class=\"mt-6 card p-6\"><div class=\"flex items-center gap-2 mb-4\">")
 		if templ_7745c5c3_Err != nil {
 			return templ_7745c5c3_Err
 		}
@@ -893,20 +953,20 @@ func AboutSection() templ.Component {
 		if templ_7745c5c3_Err != nil {
 			return templ_7745c5c3_Err
 		}
-		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 110, "<h3 class=\"text-lg font-semibold\" style=\"color: var(--text-primary)\">About</h3></div><div class=\"space-y-2 text-sm\" style=\"color: var(--text-secondary);\"><p><span class=\"font-medium uppercase tracking-wide text-xs\" style=\"color: var(--text-secondary)\">Version:</span> ")
+		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 117, "<h3 class=\"text-lg font-semibold\" style=\"color: var(--text-primary)\">About</h3></div><div class=\"space-y-2 text-sm\" style=\"color: var(--text-secondary);\"><p><span class=\"font-medium uppercase tracking-wide text-xs\" style=\"color: var(--text-secondary)\">Version:</span> ")
 		if templ_7745c5c3_Err != nil {
 			return templ_7745c5c3_Err
 		}
-		var templ_7745c5c3_Var25 string
-		templ_7745c5c3_Var25, templ_7745c5c3_Err = templ.JoinStringErrs(version.Version)
+		var templ_7745c5c3_Var28 string
+		templ_7745c5c3_Var28, templ_7745c5c3_Err = templ.JoinStringErrs(version.Version)
 		if templ_7745c5c3_Err != nil {
-			return templ.Error{Err: templ_7745c5c3_Err, FileName: `templates/admin_settings.templ`, Line: 274, Col: 134}
+			return templ.Error{Err: templ_7745c5c3_Err, FileName: `templates/admin_settings.templ`, Line: 288, Col: 134}
 		}
-		_, templ_7745c5c3_Err = templ_7745c5c3_Buffer.WriteString(templ.EscapeString(templ_7745c5c3_Var25))
+		_, templ_7745c5c3_Err = templ_7745c5c3_Buffer.WriteString(templ.EscapeString(templ_7745c5c3_Var28))
 		if templ_7745c5c3_Err != nil {
 			return templ_7745c5c3_Err
 		}
-		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 111, "</p></div></div>")
+		templ_7745c5c3_Err = templruntime.WriteString(templ_7745c5c3_Buffer, 118, "</p></div></div>")
 		if templ_7745c5c3_Err != nil {
 			return templ_7745c5c3_Err
 		}