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/components/UploadDialog.tsx

@@ -46,7 +46,7 @@ export default function UploadDialog({ onClose }: { onClose: () => void }) {
   // Every pick gets a new number, so the refusal below REMOUNTS rather than being reconciled. Two
   // over-cap picks in a row leave the node and its text identical — the message names the cap, not
   // the file — and a live region that is not touched is not announced, so the second refusal was
-  // silent to a screen reader. `KeyGate`, `SetupWizard` and `Welcome` all key their alerts for the
+  // silent to a screen reader. `SetupWizard` and `Welcome` both key their alerts for the
   // same reason, and `formAlerts.dom.test.tsx` is the test that exists because of it.
   const [pickSeq, setPickSeq] = useState(0);
   const pick_ = (f: File) => {