feat(playlists): #415 stage 1 — rotation-state schema + play ingest

First of three stages for system-playlist sample-history dedup
(Fable #415, server-side rotation per the operator's choice).

Schema (migration 0027):
- play_events.source (nullable text): which surface a play came
  from. 'for_you' / 'discover' feed rotation; NULL for library /
  user-playlist / radio / Subsonic.
- system_playlist_rotation_state(user_id, playlist_kind,
  played_track_ids uuid[], rotation_started_at, updated_at): the
  per-(user,kind) set of already-heard tracks this rotation.

Ingest:
- New RecordPlayStartedWithSource on the writer; RecordPlayStarted
  is now a thin source="" wrapper so the frozen Subsonic shim is
  untouched (no signature ripple).
- When source is a known system kind, the same txn appends the
  track to rotation state (AppendRotationPlayed keeps the array a
  set via the conflict CASE).
- /api/events play_started accepts an optional "source".

No serve-behavior change yet — Stage 2 makes shuffle prefer the
unplayed tail + resets on exhaustion; Stage 3 wires the clients to
send source and consume the rotation-aware order.

Tests: rotation appends + dedupes for a system source; source-less
play writes no rotation row. (Existing RecordPlayStarted tests are
unchanged — same wrapper signature, identical behavior at source="".)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/db/migrations/0027_system_playlist_rotation.down.sql

@@ -0,0 +1,5 @@
+-- 0027_system_playlist_rotation.down.sql
+DROP TABLE IF EXISTS system_playlist_rotation_state;
+
+ALTER TABLE play_events
+    DROP COLUMN IF EXISTS source;

internal/db/migrations/0027_system_playlist_rotation.up.sql

@@ -0,0 +1,26 @@
+-- 0027_system_playlist_rotation.up.sql — sample-history dedup for
+-- system playlists (Fable #415). Two parts:
+--
+-- 1. play_events.source: which system playlist a play originated
+--    from ('for_you' | 'discover' | future kinds), or NULL for any
+--    play not launched from a system-playlist surface (library,
+--    user playlist, radio, Subsonic). Lets the ingest path know a
+--    play should count against that playlist's rotation.
+--
+-- 2. system_playlist_rotation_state: per (user, playlist_kind) set
+--    of track ids already played in the current rotation window.
+--    Shuffle-on-play prefers the unplayed tail; when the snapshot
+--    is exhausted the set resets and rotation_started_at advances.
+--    Centralized server-side so phone / web / watch share one
+--    rotation, mirroring how radio dedup already lives server-side.
+ALTER TABLE play_events
+    ADD COLUMN source text;
+
+CREATE TABLE system_playlist_rotation_state (
+    user_id             uuid        NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    playlist_kind       text        NOT NULL,
+    played_track_ids    uuid[]      NOT NULL DEFAULT '{}',
+    rotation_started_at timestamptz NOT NULL DEFAULT now(),
+    updated_at          timestamptz NOT NULL DEFAULT now(),
+    PRIMARY KEY (user_id, playlist_kind)
+);