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/dbq/events.sql.go

@@ -54,7 +54,7 @@ func (q *Queries) GetMostRecentPlaySessionForUser(ctx context.Context, userID pg
 }
 
 const getOpenPlayEventForUser = `-- name: GetOpenPlayEventForUser :one
-SELECT id, user_id, track_id, session_id, started_at, ended_at, duration_played_ms, completion_ratio, was_skipped, client_id, session_vector_at_play, scrobbled_at FROM play_events
+SELECT id, user_id, track_id, session_id, started_at, ended_at, duration_played_ms, completion_ratio, was_skipped, client_id, session_vector_at_play, scrobbled_at, source FROM play_events
 WHERE user_id = $1 AND ended_at IS NULL
 ORDER BY started_at DESC
 LIMIT 1
@@ -78,12 +78,13 @@ func (q *Queries) GetOpenPlayEventForUser(ctx context.Context, userID pgtype.UUI
 		&i.ClientID,
 		&i.SessionVectorAtPlay,
 		&i.ScrobbledAt,
+		&i.Source,
 	)
 	return i, err
 }
 
 const getPlayEventByID = `-- name: GetPlayEventByID :one
-SELECT id, user_id, track_id, session_id, started_at, ended_at, duration_played_ms, completion_ratio, was_skipped, client_id, session_vector_at_play, scrobbled_at FROM play_events WHERE id = $1
+SELECT id, user_id, track_id, session_id, started_at, ended_at, duration_played_ms, completion_ratio, was_skipped, client_id, session_vector_at_play, scrobbled_at, source FROM play_events WHERE id = $1
 `
 
 func (q *Queries) GetPlayEventByID(ctx context.Context, id pgtype.UUID) (PlayEvent, error) {
@@ -102,15 +103,16 @@ func (q *Queries) GetPlayEventByID(ctx context.Context, id pgtype.UUID) (PlayEve
 		&i.ClientID,
 		&i.SessionVectorAtPlay,
 		&i.ScrobbledAt,
+		&i.Source,
 	)
 	return i, err
 }
 
 const insertPlayEvent = `-- name: InsertPlayEvent :one
 INSERT INTO play_events (
-    user_id, track_id, session_id, started_at, client_id
-) VALUES ($1, $2, $3, $4, $5)
-RETURNING id, user_id, track_id, session_id, started_at, ended_at, duration_played_ms, completion_ratio, was_skipped, client_id, session_vector_at_play, scrobbled_at
+    user_id, track_id, session_id, started_at, client_id, source
+) VALUES ($1, $2, $3, $4, $5, $6)
+RETURNING id, user_id, track_id, session_id, started_at, ended_at, duration_played_ms, completion_ratio, was_skipped, client_id, session_vector_at_play, scrobbled_at, source
 `
 
 type InsertPlayEventParams struct {
@@ -119,6 +121,7 @@ type InsertPlayEventParams struct {
 	SessionID pgtype.UUID
 	StartedAt pgtype.Timestamptz
 	ClientID  *string
+	Source    *string
 }
 
 func (q *Queries) InsertPlayEvent(ctx context.Context, arg InsertPlayEventParams) (PlayEvent, error) {
@@ -128,6 +131,7 @@ func (q *Queries) InsertPlayEvent(ctx context.Context, arg InsertPlayEventParams
 		arg.SessionID,
 		arg.StartedAt,
 		arg.ClientID,
+		arg.Source,
 	)
 	var i PlayEvent
 	err := row.Scan(
@@ -143,6 +147,7 @@ func (q *Queries) InsertPlayEvent(ctx context.Context, arg InsertPlayEventParams
 		&i.ClientID,
 		&i.SessionVectorAtPlay,
 		&i.ScrobbledAt,
+		&i.Source,
 	)
 	return i, err
 }
@@ -286,7 +291,7 @@ SET ended_at = $2,
     completion_ratio = $4,
     was_skipped = $5
 WHERE id = $1
-RETURNING id, user_id, track_id, session_id, started_at, ended_at, duration_played_ms, completion_ratio, was_skipped, client_id, session_vector_at_play, scrobbled_at
+RETURNING id, user_id, track_id, session_id, started_at, ended_at, duration_played_ms, completion_ratio, was_skipped, client_id, session_vector_at_play, scrobbled_at, source
 `
 
 type UpdatePlayEventEndedParams struct {
@@ -322,6 +327,7 @@ func (q *Queries) UpdatePlayEventEnded(ctx context.Context, arg UpdatePlayEventE
 		&i.ClientID,
 		&i.SessionVectorAtPlay,
 		&i.ScrobbledAt,
+		&i.Source,
 	)
 	return i, err
 }