feat(playlists): wire Refresh into PUT /api/me/timezone + registration

Completes the server side of #392 Half B.

PUT /api/me/timezone now calls scheduler.Refresh(ctx, userID) after
the DB write so the rescheduled daily-at-03:00-local job takes
effect synchronously. Failure to refresh is logged but doesn't
undo the DB write — the hourly reconciliation pass would pick it
up within an hour regardless.

POST /api/auth/register calls Refresh after successful user
insert so brand-new users get scheduled immediately rather than
waiting for the hourly pass to discover them.

system_cron.go deleted: the new scheduler subsumes its
responsibilities. The StartSystemPlaylistCron call in main.go is
also removed. Server restart now runs the new scheduler's startup
recovery + catch-up instead.

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

internal/api/auth_register.go

@@ -202,6 +202,17 @@ func (h *handlers) handleRegister(w http.ResponseWriter, r *http.Request) {
 			map[string]any{"token": usedInviteToken})
 	}
 
+	// Register the new user with the playlist scheduler so their daily
+	// builds are scheduled without waiting for the hourly reconciliation
+	// pass. Failure here is logged but doesn't fail registration — the
+	// next hourly pass will pick them up.
+	if h.playlistScheduler != nil {
+		if err := h.playlistScheduler.Refresh(r.Context(), user.ID); err != nil {
+			h.logger.Warn("api: scheduler refresh after registration",
+				"user_id", uuidToString(user.ID), "err", err)
+		}
+	}
+
 	writeJSON(w, http.StatusOK, LoginResponse{
 		Token: sessionToken,
 		User: UserView{

internal/api/me_timezone.go

@@ -56,5 +56,15 @@ func (h *handlers) handlePutTimezone(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Reschedule the user's daily build at their new timezone. Failure
+	// here doesn't undo the DB write — the hourly reconciliation pass
+	// would pick up the new tz within an hour at worst.
+	if h.playlistScheduler != nil {
+		if err := h.playlistScheduler.Refresh(r.Context(), user.ID); err != nil {
+			h.logger.Warn("api: scheduler refresh after timezone update",
+				"user_id", uuidToString(user.ID), "err", err)
+		}
+	}
+
 	w.WriteHeader(http.StatusNoContent)
 }