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.

cmd/server/main.go

@@ -179,10 +179,12 @@ func main() {
 	matchingHandler := handlers.NewMatchingHandler(queries, connManager)
 	jobsHandler := handlers.NewJobsHandler(queries, worker)
 
-	// Podcast subsystem: fetcher service + sequential download worker +
-	// poller. One instance trio per server (per-feed locks dedupe).
+	// Podcast subsystem: fetcher service + concurrent download worker
+	// (§0 item 4: pool, fair share, admin tunables) + poller. One
+	// instance trio per server (per-feed locks dedupe).
 	podcastService := services.NewPodcastService(queries)
 	podcastWorker := services.NewPodcastDownloadWorker(queries, podcastService)
+	podcastWorker.SetSettings(registry)
 	podcastHandler := handlers.NewPodcastHandler(queries, podcastService, podcastWorker)
 
 	e := echo.New()