feat(messages): end-to-end encrypted real-time direct messaging backend

Private user-to-user messages are end-to-end encrypted: the server only ever
stores ciphertext + iv and acts as a key directory (public keys) plus an opaque
store for each user's private key, wrapped client-side with a passphrase the
server never sees — so not even an admin can read a conversation. A separate
kind=system message (plaintext, no sender) powers a server-authored Siftlode
welcome shown on first open, reusable later for announcements.

- models: rework Message (kind, nullable sender/body, ciphertext+iv) + MessageKey;
  migrations 0026 (table) + 0027 (E2EE rework).
- routes/messages.py: key directory/blob endpoints, ciphertext send, conversations
  + threads (system + user), lazy welcome, all gated by require_human.
- realtime.py: in-process WebSocket connection registry; /ws delivers sent
  messages to a user's open tabs instantly (sync-callable push, single-process).

backend/app/main.py

@@ -34,6 +34,7 @@ from app.routes import (
     feed,
     health,
     me,
+    messages,
     notifications,
     playlists,
     quota,
@@ -114,6 +115,7 @@ app.include_router(tags.router)
 app.include_router(feed.router)
 app.include_router(me.router)
 app.include_router(notifications.router)
+app.include_router(messages.router)
 app.include_router(channels.router)
 app.include_router(playlists.router)
 app.include_router(admin.router)