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/main.py

@@ -44,7 +44,6 @@ from app.routes import (  # noqa: E402 — logging is configured first
     feed,
     health,
     me,
-    messages,
     notifications,
     playlists,
     plex as plex_routes,
@@ -311,7 +310,6 @@ app.include_router(me.router)
 app.include_router(diagnostics_routes.router)
 app.include_router(debug_sessions_routes.router)
 app.include_router(notifications.router)
-app.include_router(messages.router)
 app.include_router(channels.router)
 app.include_router(playlists.router)
 app.include_router(saved_views.router)