feat(playevents): #426B server — RecordOfflinePlay + /api/events play_offline

Server half of the offline-replay capture. New writer path
RecordOfflinePlay: writes a complete start+end play in one txn from
a caller-supplied `at` + duration_played_ms, applying the spec §6
skip rule (same AND-of-thresholds as RecordPlayEnded) and threading
`source` so #415 rotation advances for system-playlist plays just
like the live path. Generalizes RecordSyntheticCompletedPlay (which
hard-codes full completion). duration clamped to [0, track len].

New /api/events type "play_offline" → handleEventPlayOffline:
validates track + duration, reuses the existing req.At parse so the
play lands on the original timeline, not replay time. Subsonic
shim + live 3-call lifecycle untouched.

Flutter half next: EventsApi.playOffline, a play.offline
MutationQueue kind, and PlayEventsReporter capturing the completed
play + enqueuing it when the live calls have no server id / fail.

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

internal/api/events.go

@@ -65,6 +65,8 @@ func (h *handlers) handleEvents(w http.ResponseWriter, r *http.Request) {
 		h.handleEventPlayEnded(w, r, user, req, at)
 	case "play_skipped":
 		h.handleEventPlaySkipped(w, r, user, req, at)
+	case "play_offline":
+		h.handleEventPlayOffline(w, r, user, req, at, clientID)
 	default:
 		writeErr(w, apierror.BadRequest("bad_request", "unknown event type"))
 	}
@@ -104,6 +106,48 @@ func (h *handlers) handleEventPlayStarted(
 	})
 }
 
+// handleEventPlayOffline records a complete play that happened with
+// no/flaky connectivity, replayed from the Flutter offline mutation
+// queue (#426 part B). `at` is the original (client) play-start time
+// — parsed by handleEvents from req.At — so history lands on the
+// real timeline, not replay time. duration_played_ms drives the skip
+// rule; source advances #415 rotation for system-playlist plays.
+func (h *handlers) handleEventPlayOffline(
+	w http.ResponseWriter, r *http.Request,
+	user dbq.User, req eventRequest, at time.Time, clientID string,
+) {
+	trackID, ok := parseUUID(req.TrackID)
+	if !ok {
+		writeErr(w, apierror.BadRequest("bad_request", "invalid track_id"))
+		return
+	}
+	if req.DurationPlayedMs == nil || *req.DurationPlayedMs < 0 {
+		writeErr(w, apierror.BadRequest("bad_request", "duration_played_ms required and must be >= 0"))
+		return
+	}
+	if _, err := dbq.New(h.pool).GetTrackByID(r.Context(), trackID); err != nil {
+		if errors.Is(err, pgx.ErrNoRows) {
+			writeErr(w, &apierror.Error{Status: 404, Code: "not_found", Message: "track not found"})
+			return
+		}
+		h.logger.Error("api: events: offline lookup track", "err", err)
+		writeErr(w, apierror.InternalMsg("lookup failed", err))
+		return
+	}
+	source := ""
+	if req.Source != nil {
+		source = *req.Source
+	}
+	if err := h.events.RecordOfflinePlay(
+		r.Context(), user.ID, trackID, clientID, source, at, *req.DurationPlayedMs,
+	); err != nil {
+		h.logger.Error("api: events: play_offline", "err", err)
+		writeErr(w, apierror.InternalMsg("record failed", err))
+		return
+	}
+	writeJSON(w, http.StatusOK, okResponse{OK: true})
+}
+
 func (h *handlers) handleEventPlayEnded(
 	w http.ResponseWriter, r *http.Request,
 	user dbq.User, req eventRequest, at time.Time,