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/i18n/locales/en/errors.json

@@ -155,14 +155,8 @@
     "link_gone": "This link is no longer available",
     "link_required": "A video link or id is required",
     "media_missing": "The media file isn't on disk",
-    "message_empty": "The message is empty",
-    "message_rate": "Too many messages — slow down a moment",
-    "message_self": "You can't message yourself",
-    "message_too_long": "The message is too long",
-    "messaging_set_up": "Secure messaging is already set up",
     "name_required": "A name is required",
     "no_encryption_key": "Set TOKEN_ENCRYPTION_KEY to store secrets in the database",
-    "no_messaging": "That member hasn't set up messaging yet",
     "no_profile": "There's no download profile to use",
     "no_subscription_id": "There's no YouTube subscription on record yet — sync your subscriptions first",
     "not_authenticated": "You're not signed in",