fix: brainstorm infinite loop, ghost cursor, embedding ::vector cast, semantic search, billing stats, usage meter accordion

- Fix useBrainstormSocket: stable guestId via useRef, remove setState in cleanup
- Fix GhostCursor: direct DOM manipulation via refs, no useState re-renders
- Fix all SQL embedding queries: add ::vector cast on text columns
- Fix embedding truncation to 15000 chars (under 8192 token limit)
- Fix NoteEmbedding INSERT: remove non-existent updatedAt column
- Fix billing page: show all quota stats in grid instead of single metric
- Fix usage meter: accordion expand/collapse, per-feature detail
- Fix semantic search: rebuild 103 note embeddings, ::vector cast on vectorSearch
- Fix brainstorm expand/manual-idea/create: ::vector cast on embedding SQL

memento-note/hooks/use-brainstorm-socket.ts

@@ -32,7 +32,11 @@ export function useBrainstormSocket(
   const [activities, setActivities] = useState<ActivityEvent[]>([])
   const [aiProcessingNodeId, setAiProcessingNodeId] = useState<string | null>(null)
 
-  const effectiveUserId = userId || `guest_${Math.random().toString(36).slice(2, 10)}`
+  const guestIdRef = useRef<string | null>(null)
+  if (!guestIdRef.current && !userId) {
+    guestIdRef.current = `guest_${Math.random().toString(36).slice(2, 10)}`
+  }
+  const effectiveUserId = userId || guestIdRef.current!
 
   useEffect(() => {
     if (!sessionId) return
@@ -85,8 +89,6 @@ export function useBrainstormSocket(
     return () => {
       socket.disconnect()
       socketRef.current = null
-      setOthers([])
-      setAiProcessingNodeId(null)
     }
   }, [sessionId, effectiveUserId, userName])