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.

internal/handlers/system_settings.go

@@ -178,6 +178,21 @@ func validateSettingValue(def database.SettingDefault, value string) error {
 		if _, err := strconv.ParseBool(value); err != nil {
 			return fmt.Errorf("value must be true or false")
 		}
+	case database.SettingTypeFloat:
+		f, err := strconv.ParseFloat(value, 64)
+		if err != nil {
+			return fmt.Errorf("value must be a number")
+		}
+		if def.Min != "" {
+			if mn, err := strconv.ParseFloat(def.Min, 64); err == nil && f < mn {
+				return fmt.Errorf("value must be >= %s", def.Min)
+			}
+		}
+		if def.Max != "" {
+			if mx, err := strconv.ParseFloat(def.Max, 64); err == nil && f > mx {
+				return fmt.Errorf("value must be <= %s", def.Max)
+			}
+		}
 	case database.SettingTypeString:
 		if value == "" {
 			return fmt.Errorf("value must not be empty")