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.test.ts

@@ -26,8 +26,6 @@ describe("qk — bare vs parameterized branch", () => {
     expect(qk.playlist(7)).toEqual(["playlist", 7]);
     expect(qk.ytSearch(null, 3)).toEqual(["yt-search", null, 3]);
     expect(qk.ytSearch("cats", 3)).toEqual(["yt-search", "cats", 3]);
-    expect(qk.thread(null)).toEqual(["thread", null]);
-    expect(qk.thread(42)).toEqual(["thread", 42]);
   });
 });
 
@@ -50,7 +48,6 @@ describe("qk — single-id and scalar keys", () => {
     expect(qk.myStatus()).toEqual(["my-status"]);
     expect(qk.channels()).toEqual(["channels"]);
     expect(qk.playlists()).toEqual(["playlists"]);
-    expect(qk.conversations()).toEqual(["conversations"]);
     expect(qk.downloads()).toEqual(["downloads"]);
     expect(qk.adminUsers()).toEqual(["admin-users"]);
     expect(qk.plex()).toEqual(["plex"]);