fix(security): comprehensive security hardening — TLS, HMAC WS auth, rate limiting, IP leak prevention

CRITICAL fixes:
- Auto-generated self-signed TLS certs (HTTPS/WSS by default)
- Removed session_key from /srp/verify response (was sent in plaintext)
- Replaced with HMAC-SHA256 ws_token for WebSocket authentication

HIGH fixes:
- WebSocket auth now validates ws_token via hmac.compare_digest()
- /clear endpoint requires Bearer admin_token (printed at server start)
- Password no longer required as CLI arg — supports env var + getpass prompt
- Removed user_ip from Message model (no longer broadcast to clients)

MEDIUM fixes:
- Rate limiter on /srp/init and /srp/verify (10 req/min/IP)
- MessageStore capped at 1000 messages (prevents RAM DoS)
- access_log disabled (was leaking request metadata)

LOW fixes:
- Username sanitization against rich markup injection
- Dead code removed from helpers.py

All 79 tests passing.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cmd_chat/server/models.py

@@ -11,7 +11,6 @@ class Message:
     timestamp: str = field(
         default_factory=lambda: datetime.now(timezone.utc).isoformat()
     )
-    user_ip: str = ""
     username: str = ""