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/database/settings_registry.go

@@ -25,6 +25,7 @@ import (
 // SettingType enumerates the value types stored in system_settings.setting_type.
 const (
 	SettingTypeInt        = "int"
+	SettingTypeFloat      = "float"
 	SettingTypeBool       = "bool"
 	SettingTypeString     = "string"
 	SettingTypeStringList = "string_list"
@@ -83,6 +84,8 @@ var SettingDefaults = []SettingDefault{
 	{Key: "worker_pool_size", Value: "3", Type: SettingTypeInt, Min: "1", Max: "100", RequiresRestart: true, Category: "performance", Group: "Worker Pool", Description: "Number of background worker goroutines"},
 	{Key: "worker_queue_cap", Value: "100", Type: SettingTypeInt, Min: "1", Max: "10000", RequiresRestart: true, Category: "performance", Group: "Worker Pool", Description: "Background worker job queue capacity"},
 	{Key: "podcast_poll_interval_seconds", Value: "3600", Type: SettingTypeInt, Min: "300", Max: "604800", Category: "podcasts", Group: "Podcasts", Description: "How often to poll podcast feeds for new episodes (seconds; complete feeds back off)"},
+	{Key: "podcast_download_concurrency", Value: "4", Type: SettingTypeInt, Min: "1", Max: "16", Category: "podcasts", Group: "Podcasts", Description: "Podcast episode downloads in flight server-wide (the connection is the limit; fair-share spreads them across feeds)"},
+	{Key: "podcast_download_speed_mbps", Value: "0", Type: SettingTypeFloat, Min: "0", Max: "10000", Category: "podcasts", Group: "Podcasts", Description: "Shared bandwidth cap for podcast downloads in megabits/second (0 = unlimited)"},
 }
 
 // defaultBy indexes SettingDefaults by key for O(1) lookup.
@@ -173,6 +176,26 @@ func (r *SettingsRegistry) getInt(key string) int {
 	return n
 }
 
+func (r *SettingsRegistry) getFloat(key string) float64 {
+	d := defaultBy[key]
+	v := r.raw(key)
+	f, err := strconv.ParseFloat(v, 64)
+	if err != nil {
+		f, _ = strconv.ParseFloat(d.Value, 64)
+	}
+	if d.Min != "" {
+		if mn, err := strconv.ParseFloat(d.Min, 64); err == nil && f < mn {
+			f = mn
+		}
+	}
+	if d.Max != "" {
+		if mx, err := strconv.ParseFloat(d.Max, 64); err == nil && f > mx {
+			f = mx
+		}
+	}
+	return f
+}
+
 func (r *SettingsRegistry) getBool(key string) bool {
 	v := r.raw(key)
 	b, err := strconv.ParseBool(v)
@@ -189,13 +212,24 @@ func (r *SettingsRegistry) ScanPollInterval() time.Duration {
 	return time.Duration(r.getInt("scan_poll_interval_seconds")) * time.Second
 }
 
-// PodcastPollInterval is the feed-poller tick (live setting; the poller
-// re-reads it every cycle — writes go through the settings API path,
-// AGENTS.md gotcha #2).
+// PodcastPollInterval is how often the poller checks feeds, as a
+// duration (live setting; the poller re-reads it every cycle — writes
+// go through the settings API path, AGENTS.md gotcha #2).
 func (r *SettingsRegistry) PodcastPollInterval() time.Duration {
 	return time.Duration(r.getInt("podcast_poll_interval_seconds")) * time.Second
 }
 
+// PodcastDownloadConcurrency: episode downloads in flight server-wide
+// (the connection is the limit; fair-share spreads them across feeds).
+func (r *SettingsRegistry) PodcastDownloadConcurrency() int {
+	return r.getInt("podcast_download_concurrency")
+}
+
+// PodcastDownloadSpeedMbps: shared bandwidth cap (0 = unlimited).
+func (r *SettingsRegistry) PodcastDownloadSpeedMbps() float64 {
+	return r.getFloat("podcast_download_speed_mbps")
+}
+
 // AutoScanEnabled reports whether auto-scanning is on.
 func (r *SettingsRegistry) AutoScanEnabled() bool { return r.getBool("auto_scan_enabled") }