Fix OPK loss on SPK-grace retry, deletion sync, add keys_updated push

- chat_core: defer one-time-prekey deletion until the first message
  decrypts successfully; deleting it on load made the SPK grace-period
  retry derive a wrong shared secret and lose the message permanently
- chat_core: fix get_deleted_since params (since -> since_ts) and
  response field (message_ids -> deleted_ids) so incremental deletion
  sync actually works
- chat_core: route keys_updated pushes into the notification queue
- server: notify contacts with keys_updated when a user uploads a new
  SPK or logs in with a new device, so clients invalidate cached key
  bundles instead of waiting for the TTL
- server: rate-limit download_stream like other heavy handlers

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

server.py

@@ -842,6 +842,10 @@ async def handle_login_finish(msg: dict, writer: ProtocolWriter, state: dict) ->
             "ip": addr,
             "added_at": datetime.now(timezone.utc).isoformat(),
         }, exclude_writer=writer)
+        # Contacts must refetch key bundles to include the new device
+        if contacts:
+            await _notify_users(contacts, "keys_updated", {"user_id": user_id},
+                                exclude_writer=writer)
 
     return {"user_id": user_id, "username": user["username"], "email": user["email"],
             "device_id": device_id}
@@ -1060,6 +1064,13 @@ async def handle_ensure_prekeys(msg: dict, session: dict, writer: ProtocolWriter
                     {"count": count, "spk_created_at": spk_created_at,
                      "uploaded_spk": uploaded_spk, "uploaded_otps": uploaded_otps})
 
+    # SPK change invalidates key bundles cached by other clients
+    if uploaded_spk:
+        contacts = await adb.get_user_contacts(user_id)
+        if contacts:
+            await _notify_users(contacts, "keys_updated", {"user_id": user_id},
+                                exclude_writer=writer)
+
 
 async def handle_rotate_keys(msg: dict, session: dict, writer: ProtocolWriter):
     if await _is_rate_limited(f"rotate_keys|{session['user_id']}", 3):
@@ -2478,6 +2489,10 @@ async def handle_download_stream(msg: dict, session: dict, writer: ProtocolWrite
     so the client can reassemble chunks in order even if delivery is reordered.
     """
     file_id = msg.get("file_id", "")
+    if await _is_rate_limited(f"download_stream|{session['user_id']}", 30):
+        await send_resp(msg, writer, "download_stream", "error",
+                        {"message": "Too many requests. Try later."})
+        return
     result = await _validate_download(msg, session, writer, "download_stream")
     if not result:
         return