docs(dev): websocket wire doc rewritten from source; endpoint doc ping claim fixed

The Feb-era 700-line page documented client->server JSON messages the
server never acted on (handleMessage logs only), a wrong ping/pong
exchange, and missed every message type added since. Rewritten tight
(~100 lines), every claim pinned: dual auth (JWT ?token= or device
Bearer header), initial_state on connect under the connecting
identity's RLS scope, the full 12-type message catalog (including
collection_updated, found per-user-targeted in the collections handler
and defined as a raw string outside the sync constants), WS-frame
keepalive (30s server ping, 90s read deadline — browsers answer
automatically), and the backpressure contract clients must honor:
bounded per-connection queues skip messages when full, so the socket
is a hint and REST refetch + fresh initial_state is the gap-healer.
Fan-out documented precisely: sync/scan events instance-wide,
collection events per-user.

Also corrected the gated endpoint doc (api/websocket/protocol.md):
its 'client sends {type:ping}, server responds pong' section
contradicted the handler; replaced with the true keepalive story.
Route pin untouched — the drift gates stay green.

docs/developer/api/websocket/protocol.md

@@ -24,15 +24,16 @@ All messages are JSON objects with a `type` field.
 
 ### Client → Server Messages
 
-#### Ping (Heartbeat)
+None are interpreted. The socket is a one-directional push feed — the
+server logs (but ignores) anything a client sends. All reads and writes
+go through the REST API.
 
-```json
-{
-  "type": "ping"
-}
-```
+#### Keepalive
 
-Keep connection alive. Server responds with `pong`.
+The server sends a WebSocket **ping frame every 30 seconds**; a
+connection that misses pongs for 90 seconds is removed. Standard
+WebSocket clients (browsers included) answer pongs automatically —
+there is nothing to implement.
 
 ### Server → Client Messages