fix(kuma-bridge): a rejected login is not-ready (was falsely 'up'); clearer connect vs login errors

Verified with a reachable Kuma and deliberately wrong credentials:
/ready -> 503 'Kuma login rejected: Incorrect username or password.'

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docker/kuma-bridge/app.py

@@ -60,7 +60,9 @@ def _connect() -> UptimeKumaApi:
                 api.disconnect()
             except Exception:  # noqa: BLE001, S110
                 pass
-            raise HTTPException(status_code=503, detail=f"Kuma unreachable: {exc}") from exc
+            # Distinguish "cannot reach" from "rejected us" — very different fixes.
+            kind = "login rejected" if "username or password" in str(exc).lower() else "unreachable"
+            raise HTTPException(status_code=503, detail=f"Kuma {kind}: {exc}") from exc
         _api = api
 
     return _api
@@ -151,15 +153,18 @@ def _kuma_state() -> tuple[str, str | None]:
     except Exception as exc:  # noqa: BLE001 — connection refused/DNS/timeout
         return "down", f"HTTP unreachable: {exc}"
 
+    # An authenticated, connected session is the real proof: _connect() performs
+    # the login round-trip, so bad credentials / missing 2FA / an incompatible
+    # login API all surface here as "down" rather than a false "up".
     try:
-        _call(lambda api: _require_live(api) and api.get_monitor_status(0))
+        with _lock:
+            api = _connect()
+            if not getattr(api, "sio", None) or not api.sio.connected:
+                _reset()
+                return "down", "Kuma socket not connected"
         return "up", None
     except HTTPException as exc:
-        # A live call on a bogus id may error while the session is fine — the
-        # socket being connected is what we actually care about here.
-        if "not connected" in str(exc.detail):
         return "down", str(exc.detail)
-        return "up", None
     except Exception as exc:  # noqa: BLE001 — must never 500
         return "down", str(exc)