feat!: remove direct messaging (chat)

E2EE direct messages, the chat dock, the Messages module, the message
WebSocket and its realtime registry, and the messages / message_keys
tables (migration 0097; the downgrade restores the empty schema). Prod
held two welcome notes, no message between people and one key — never
used (owner decision, E34). WebSocket now has no owner module: the lint
bans it everywhere. Privacy policy updated: it no longer promises E2EE
for a feature that is gone.

backend/tests/test_route_conventions.py

@@ -23,11 +23,9 @@ COMMITTING_HELPERS = {
     "search.py:_ingest_candidates": "stages around YouTube calls: the stubs must survive an enrich "
     "that fails",
     "search.py:_classify_shorts": "the last stage of that ingest, after its probes",
-    "auth.py:upsert_pending_invite": "commits only when it writes — the contract its three "
+    "auth.py:upsert_pending_invite": "commits only when it writes — the contract its two "
     "callers rely on (`_register_account` persists its own flushed user because a no-op here "
     "commits nothing)",
-    "messages.py:ensure_welcome": "the websocket push after it must announce a row the client "
-    "can already fetch",
     "debug_sessions.py:_end_send": "gives back the claim a send took, in a `finally`: it runs when "
     "the handler is already unwinding and has no commit left to reach",
 }