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/networkBoundary.contract.test.ts

@@ -34,7 +34,6 @@ describe("network and console boundary", () => {
   it.each([
     ["src/lib/http.ts", "fetch"],
     ["src/lib/api/core.ts", "XMLHttpRequest"],
-    ["src/lib/messagesSocket.ts", "WebSocket"],
   ])("%s may use %s — and nothing else", async (owner, own) => {
     const found = await flagged(ALL, owner);
     // Its own global goes quiet; the other two globals, the two properties and console still fire.