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/app/logs.py

@@ -688,7 +688,6 @@ _POLLED_ROUTES = frozenset(
     {
         "/api/me",
         "/api/me/notifications/unread_count",
-        "/api/messages/unread_count",
         "/api/downloads",
         "/api/downloads/index",
         "/api/downloads/usage",