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.

database/schema/schema.sql

@@ -102,7 +102,9 @@ INSERT INTO system_settings (setting_key, setting_value, description, setting_ty
 ('worker_pool_size', '3', 'Number of background worker goroutines', 'int', '1', '100', TRUE, 'performance'),
 ('worker_queue_cap', '100', 'Background worker job queue capacity', 'int', '1', '10000', TRUE, 'performance'),
 -- podcasts
-('podcast_poll_interval_seconds', '3600', 'How often to poll podcast feeds for new episodes (seconds; complete feeds back off)', 'int', '300', '604800', FALSE, 'podcasts')
+('podcast_poll_interval_seconds', '3600', 'How often to poll podcast feeds for new episodes (seconds; complete feeds back off)', 'int', '300', '604800', FALSE, 'podcasts'),
+('podcast_download_concurrency', '4', 'Podcast episode downloads in flight server-wide (the connection is the limit; fair-share spreads them across feeds)', 'int', '1', '16', FALSE, 'podcasts'),
+('podcast_download_speed_mbps', '0', 'Shared bandwidth cap for podcast downloads in megabits/second (0 = unlimited)', 'float', '0', '10000', FALSE, 'podcasts')
 ON CONFLICT (setting_key) DO NOTHING;
 
 -- Create refresh_tokens table
@@ -1989,7 +1991,16 @@ ALTER TABLE media_items
     -- list renders this inline (D8/D19) — never Processing Issues.
     ADD COLUMN IF NOT EXISTS download_state VARCHAR(20) NOT NULL DEFAULT 'pending',
     ADD COLUMN IF NOT EXISTS download_error TEXT,
-    ADD COLUMN IF NOT EXISTS download_attempts INT NOT NULL DEFAULT 0;
+    ADD COLUMN IF NOT EXISTS download_attempts INT NOT NULL DEFAULT 0,
+    -- Monotonic queue ticket (Postgres SEQUENCE nextval at enqueue
+    -- time): the download pool claims in ticket order, so the enqueue
+    -- loop's ordering IS the download order (effective listening order
+    -- — §0 items 4/5). NULL for rows never queued.
+    ADD COLUMN IF NOT EXISTS download_queue_pos BIGINT;
+
+-- Queue ticket source: atomic across concurrent enqueues (per-feed
+-- locks serialize within a feed; the sequence covers across feeds).
+CREATE SEQUENCE IF NOT EXISTS podcast_queue_seq;
 
 -- Identity + ordering + worker-worklist indexes (all partial: scan
 -- rows never touch these columns).
@@ -1999,6 +2010,8 @@ CREATE INDEX IF NOT EXISTS idx_media_items_feed_pos
     ON media_items(source_feed_id, feed_position) WHERE source_feed_id IS NOT NULL;
 CREATE INDEX IF NOT EXISTS idx_media_items_feed_undownloaded
     ON media_items(source_feed_id) WHERE source_feed_id IS NOT NULL AND download_state <> 'downloaded';
+CREATE INDEX IF NOT EXISTS idx_media_items_queue
+    ON media_items(download_queue_pos) WHERE source = 'podcast_episode' AND download_state = 'queued';
 
 
 -- ============================================================