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/worker.py

@@ -11,7 +11,7 @@ Design:
     instantly (no re-download); a fresh asset is claimed pending→downloading by exactly one
     worker (a DB compare-and-set), so two jobs for the same video+format never double-download.
   * Live progress is written to the job row (short-lived sessions, throttled ~1/s) — the
-    frontend polls it; the worker can't reach the API's in-process WebSocket registry.
+    frontend polls it (the worker is a separate process with no channel to the browser).
   * Pause/cancel are cooperative: the progress hook re-reads the job status and aborts the
     yt-dlp download if it's no longer `running`.
   * Crash-safe: on startup, orphaned `running` jobs are requeued and `downloading` assets reset.