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(