feat(#392): publish quarantine + playlist mutation events

Slice 3a — extends the producer set onto the SSE bus.

quarantine events (5 producer sites):
- quarantine.flagged (user-side flag): broadcast so the flagger's other
  clients invalidate their Hidden tab and admins' clients invalidate
  their queue.
- quarantine.unflagged (user-side unflag): scoped to the user.
- quarantine.resolved / .file_deleted / .deleted_via_lidarr (admin
  actions): broadcast because a single admin action can affect every
  user who'd flagged that track.

playlist events (6 producer sites):
- playlist.created / .updated / .deleted: owner-scoped.
- playlist.tracks_changed: emitted for AppendTracks / RemoveTrack /
  Reorder. Owner-scoped. Single kind for all three mutations because
  the client invalidation logic is identical (refetch the detail).

Public-playlist subscribers (other users viewing someone else's public
playlist) intentionally left out — needs a separate broadcast kind, not
exercised yet at single-household scale.

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

internal/api/quarantine.go

@@ -67,6 +67,9 @@ func (h *handlers) handleFlag(w http.ResponseWriter, r *http.Request) {
 		}
 		return
 	}
+	// Broadcast: the flagging user's other clients invalidate their
+	// Hidden tab; admins' clients invalidate their quarantine queue.
+	h.publishQuarantineEvent("quarantine.flagged", user.ID, trackID, true)
 	writeJSON(w, http.StatusCreated, quarantineViewFrom(row))
 }
 
@@ -89,6 +92,8 @@ func (h *handlers) handleUnflag(w http.ResponseWriter, r *http.Request) {
 		writeErr(w, apierror.InternalMsg("unflag failed", err))
 		return
 	}
+	// Scoped to user: only their other clients need to invalidate.
+	h.publishQuarantineEvent("quarantine.unflagged", user.ID, id, false)
 	w.WriteHeader(http.StatusNoContent)
 }