feat: Add E2E encryption support to Matrix bot

- matrix-nio[e2e] with libolm for Megolm encryption
- Persistent crypto store volume for key persistence
- Auto-accept key verification (SAS)
- Upload device keys on first login

CF-1147

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -11,3 +11,8 @@ services:
     command: python bot.py
     env_file: .env
     restart: unless-stopped
+    volumes:
+      - bot-crypto:/data/crypto_store
+
+volumes:
+  bot-crypto: