Fix StatusUpdate field annotation and is_online batch-call mismatch

- status_message: str | Field(...) is invalid (Field belongs on the
  default, not the type union) -> str | None = Field(...)
- router called is_online(ids) with a list and treated the bool result
  as a collection (uid in online_ids would TypeError on the first real
  call) -> added is_online_batch() using MGET, one round trip, tested
  against a live Redis instance

backend/app/routers/presence.py

@@ -43,7 +43,7 @@ class PresenceResponse(BaseModel):
 
 
 class StatusUpdate(BaseModel):
-    status_message: str | Field(default=None, max_length=_STATUS_MAX_LEN)
+    status_message: str | None = Field(default=None, max_length=_STATUS_MAX_LEN)
 
     @field_validator("status_message")
     @classmethod
@@ -107,7 +107,7 @@ async def get_presence(
         )
 
     # Check Redis for online status
-    online_ids = await _presence_store.is_online(ids)
+    online_ids = await _presence_store.is_online_batch(ids)
 
     # Fetch DB records for last_seen_at and status_message
     result = await db.execute(

backend/app/services/presence.py

@@ -95,6 +95,16 @@ class PresenceStore:
         exists = await self.client.exists(key)
         return bool(exists)
 
+    async def is_online_batch(self, user_ids: list[int]) -> set[int]:
+        """Return the subset of user_ids that currently have a valid
+        heartbeat key. Uses MGET (one round trip) rather than one EXISTS
+        per user."""
+        if not user_ids:
+            return set()
+        keys = [self._heartbeat_key(uid) for uid in user_ids]
+        values = await self.client.mget(keys)
+        return {uid for uid, v in zip(user_ids, values) if v is not None}
+
     # ------------------------------------------------------------------
     # Presence lookup
     # ------------------------------------------------------------------