fix(realtime): keep socket through reconnects, stop re-subscribe storm

Two correctness bugs in the real-time stack — both silent failures, both
session-wide once they trigger.

(1) `SocketProvider` was setting the React context to null on every
`disconnect` event. socket.io's built-in reconnection re-establishes the
underlying transport and replays handlers, but the React tree had
already lost its reference to the socket — so every `useSocket()`
consumer saw null until a session/port change forced a remount. Effect:
after the first transient drop (laptop sleep, wifi blip, server
restart), realtime invalidation and toasts went dead session-wide with
no user-visible signal.

Fix: keep the socket reference stable for the lifetime of the
session+port, and surface a separate `isConnected` boolean for any UI
that wants to render an offline indicator. Exposed as a new
`useIsSocketConnected()` hook; `useSocket()` signature is unchanged.

(2) `useRealtimeInvalidation` captured `eventMap` as a useEffect
dependency. Every caller passes a fresh `{ ... }` object literal on each
render, so the effect re-ran every render → `socket.off`/`socket.on`
storm on pages with many subscribed events.

Fix: extract the subscription logic into a pure helper
(`realtime-invalidation-core.ts`, JSX-free for vitest). The hook now
keeps the latest map in a ref and only re-subscribes when the SET of
event names changes (joined-keys signature, not object identity). The
handler reads `ref.current` at fire time, so callers still see fresh
queryKey lists without re-binding.

Helper is unit-tested with a stub socket: registration count,
fire-time map lookup, cleanup deregistration, missing-event safety.

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

src/hooks/use-realtime-invalidation.ts

@@ -1,14 +1,20 @@
 'use client';
 
-import { useEffect } from 'react';
-import { useQueryClient, type QueryKey } from '@tanstack/react-query';
+import { useEffect, useRef } from 'react';
+import { useQueryClient } from '@tanstack/react-query';
 
 import { useSocket } from '@/providers/socket-provider';
+import { subscribeRealtimeInvalidations, type EventMap } from '@/hooks/realtime-invalidation-core';
+
+// Re-export for convenience so callers don't need to know about the split.
+export type { EventMap, SocketLike } from '@/hooks/realtime-invalidation-core';
 
 /**
  * Subscribes to socket events and invalidates React Query caches.
  *
- * @param eventMap - Maps socket event names to arrays of query keys to invalidate.
+ * Safe to call with an inline-literal `eventMap` — the hook only re-subscribes
+ * when the SET of event keys actually changes (not when the object identity
+ * changes). The latest query-key list is read at event fire-time via a ref.
  *
  * @example
  * useRealtimeInvalidation({
@@ -17,31 +23,29 @@ import { useSocket } from '@/providers/socket-provider';
  *   'client:archived': [['clients']],
  * });
  */
-export function useRealtimeInvalidation(
-  eventMap: Record<string, QueryKey[]>,
-) {
+export function useRealtimeInvalidation(eventMap: EventMap) {
   const socket = useSocket();
   const queryClient = useQueryClient();
 
+  // Stash the latest map in a ref so handlers always see fresh queryKeys
+  // without re-subscribing.
+  const eventMapRef = useRef(eventMap);
+  eventMapRef.current = eventMap;
+
+  // Re-subscribe ONLY when the set of event names changes. Object identity
+  // of `eventMap` flips on every caller render; the joined key signature
+  // doesn't.
+  const eventKeysSig = Object.keys(eventMap).sort().join('|');
+
   useEffect(() => {
     if (!socket) return;
-
-    const handlers: Array<{ event: string; handler: (...args: unknown[]) => void }> = [];
-
-    for (const [event, queryKeys] of Object.entries(eventMap)) {
-      const handler = () => {
-        for (const key of queryKeys) {
-          queryClient.invalidateQueries({ queryKey: key });
-        }
-      };
-      socket.on(event, handler);
-      handlers.push({ event, handler });
-    }
-
-    return () => {
-      for (const { event, handler } of handlers) {
-        socket.off(event, handler);
-      }
-    };
-  }, [socket, queryClient, eventMap]);
+    // eventMapRef is intentionally not in deps — it's a ref; we only want to
+    // re-run when the socket, queryClient, or the event-key SET changes.
+    return subscribeRealtimeInvalidations(
+      socket,
+      eventKeysSig.length > 0 ? eventKeysSig.split('|') : [],
+      queryClient,
+      () => eventMapRef.current,
+    );
+  }, [socket, queryClient, eventKeysSig]);
 }