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/api/events.go

@@ -20,6 +20,10 @@ type eventRequest struct {
 	PositionMs       *int32  `json:"position_ms"`
 	At               *string `json:"at"`
 	ClientID         *string `json:"client_id"`
+	// Source tags which surface a play_started came from. "for_you"
+	// / "discover" feed the system-playlist rotation dedup (#415);
+	// absent / "" for library, user-playlist, radio, Subsonic.
+	Source *string `json:"source"`
 }
 
 type playStartedResponse struct {
@@ -84,7 +88,11 @@ func (h *handlers) handleEventPlayStarted(
 		writeErr(w, apierror.InternalMsg("lookup failed", err))
 		return
 	}
-	res, err := h.events.RecordPlayStarted(r.Context(), user.ID, trackID, clientID, at)
+	source := ""
+	if req.Source != nil {
+		source = *req.Source
+	}
+	res, err := h.events.RecordPlayStartedWithSource(r.Context(), user.ID, trackID, clientID, source, at)
 	if err != nil {
 		h.logger.Error("api: events: play_started", "err", err)
 		writeErr(w, apierror.InternalMsg("record failed", err))