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/README.md

@@ -21,7 +21,7 @@ Three layers, by *who* runs the code — not by feature:
    `issues/bundle.py` is the one definition of a bug report's files, for the zip export and the tracker.
 
 3. **Flat `app/*.py` core modules — cross-cutting primitives.** `auth`, `sysconfig`, `quota`, `audit`,
-   `notifications`, `userscope`, `realtime`, `ratelimit`, `progress`, `logs` (the one logging
+   `notifications`, `userscope`, `ratelimit`, `progress`, `logs` (the one logging
    configuration), `logsink` (its `log_events` table), `diagnostics` (a user's consented log
    level), `debugsession` (a recording that tags its user's lines, E16) and `collation` (A→Z in the
    reader's alphabet, E30), … are single-file primitives many layers import. They depend on nothing above them (a core