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/lib/queryKeys.ts

@@ -62,12 +62,7 @@ export const qk = {
   notifications: () => ["notifications"] as const,
   notifUnread: () => ["notif-unread"] as const,
 
-  // --- messaging ---
-  conversations: () => ["conversations"] as const,
-  thread: (partnerId: number | null) => ["thread", partnerId] as const,
-  messageKey: () => ["message-key"] as const,
-  messageUnread: () => ["message-unread"] as const,
-  messageUsers: () => ["message-users"] as const,
+  // --- sharing ---
   shareRecipients: () => ["share-recipients"] as const,
 
   // --- downloads ---
@@ -168,6 +163,5 @@ export const LANGUAGE_ORDERED = [
   qk.discoveredChannels(),
   qk.blockedChannels(),
   qk.shareRecipients(),
-  qk.messageUsers(),
   qk.plexCollections(),
 ] as const;