fix: return 401 (not 500) when bcrypt hash is malformed (#21)

- verify_password catches ValueError from passlib so a mangled hash
  ($ signs stripped by shell/Docker) returns False instead of crashing
- Settings.check_password_hash logs a clear startup error with fix
  instructions when AUTH_PASSWORD_HASH doesn't start with '$2'

backend/tests/test_auth.py

@@ -56,3 +56,15 @@ async def test_service_key_disabled_when_not_configured(client: AsyncClient):
     settings.mcp_service_key = ""
     res = await client.get("/api/v1/nodes", headers={"X-MCP-Service-Key": "any-key"})
     assert res.status_code == 401
+
+
+async def test_login_with_malformed_hash_returns_401_not_500(client: AsyncClient):
+    """Malformed hash (e.g. $ stripped by shell) must not crash with 500."""
+    from app.core.config import settings
+    original = settings.auth_password_hash
+    settings.auth_password_hash = "2b12RtMbyw17l4N5UGzeXMNAWu"  # $ signs stripped
+    try:
+        res = await client.post("/api/v1/auth/login", json={"username": "admin", "password": "admin"})
+        assert res.status_code == 401
+    finally:
+        settings.auth_password_hash = original