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.

frontend/src/components/ui/README.md

@@ -30,9 +30,8 @@ The direction is one-way from below, too: `lib/` may not import from `components
 included, not even a type (same lint file). A contract both layers need lives in `lib/`: the `Column`
 / `FilterMap` shape and `filterRows` are `lib/tableColumns`, which `tableHeader` and `DataTable` import.
 
-Counter-examples, deliberately NOT here: `PosterTile` (knows about Plex montage posters), `KeyGate`
-(secure-messaging key setup), `plexCards`, `downloadRow`, `channelColumns` — all shared, none
-domain-free.
+Counter-examples, deliberately NOT here: `PosterTile` (knows about Plex montage posters),
+`plexCards`, `downloadRow`, `channelColumns` — all shared, none domain-free.
 
 ## What lives here