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;