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/api/facade.test.ts

@@ -30,7 +30,6 @@ const SLICE_MODULES = [
   ["quotaApi", () => import("./quota")],
   ["adminApi", () => import("./admin")],
   ["plexApi", () => import("./plex")],
-  ["messagingApi", () => import("./messaging")],
   ["downloadsApi", () => import("./downloads")],
   ["diagnosticsApi", () => import("./diagnostics")],
   ["debugSessionsApi", () => import("./diagnostics")],