feat(presence): Redis-backed presence for multi-replica prod

Extract a PresencePort seam (async setFocus/clearSocket/isViewing) with two
impls: the existing in-memory Map (dev, single instance) and a new
RedisPresenceService (prod). MessageModule provides PRESENCE_PORT via a
REDIS_URL-gated factory — same pattern as the socket.io Redis adapter — so
'who is viewing what' is shared across replicas and the notification projector's
presence gate works at N>1. Gateway + projector inject the port; isViewing is
now awaited. Presence spec updated to async (passing).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/iios-service/src/notifications/presence.port.ts

@@ -0,0 +1,15 @@
+/**
+ * Presence seam: tracks which thread each socket has in the foreground so the notification
+ * projector can suppress push for a thread the recipient is actively viewing. Async so it can be
+ * backed by Redis across replicas (prod) or an in-memory Map on a single instance (dev).
+ */
+export interface PresencePort {
+  /** Record a socket's foregrounded thread (null when the window is blurred / no thread open). */
+  setFocus(socketId: string, userId: string, threadId: string | null): Promise<void>;
+  /** Forget everything about a socket (on disconnect). */
+  clearSocket(socketId: string): Promise<void>;
+  /** True if ANY of the user's sockets currently has this thread in the foreground. */
+  isViewing(userId: string, threadId: string): Promise<boolean>;
+}
+
+export const PRESENCE_PORT = Symbol('PRESENCE_PORT');