fix(gateway): drop app.state.config snapshot and freeze run_events_config

Address @ShenAC-SAC's BUG-001 review on #3131. The previous cut still
stored an ``AppConfig`` snapshot on ``app.state.config`` for startup
bootstrap. Two follow-on hazards from that:

1. Future code touching the gateway lifespan could accidentally start
   reading ``app.state.config`` again, silently regressing the request
   hot path back to a stale snapshot.
2. ``get_run_context()`` paired a freshly-reloaded ``AppConfig`` with the
   startup-bound ``event_store`` and a *live* ``run_events_config``
   field — so an operator who edited ``run_events.backend`` mid-flight
   would have produced a run context whose ``event_store`` and
   ``run_events_config`` referred to different backends.

Clean approach (aligned with the direction in PR #3128):

- ``lifespan()`` keeps a local ``startup_config`` variable and passes it
  explicitly into ``langgraph_runtime(app, startup_config)`` and into
  ``start_channel_service``. No ``app.state.config`` attribute is set at
  any point.
- ``langgraph_runtime`` now accepts ``startup_config`` as a required
  parameter, removing the ``getattr(app.state, "config", None)`` lookup
  and the "config not initialised" runtime error.
- The matching ``run_events_config`` is frozen onto ``app.state`` next
  to ``run_event_store`` so ``get_run_context`` reads the two from the
  same startup-time source. ``app_config`` continues to be resolved
  live via ``get_app_config()``.
- ``backend/CLAUDE.md`` boundary explanation updated to spell out the
  ``startup_config`` / ``get_app_config()`` split.

New regression test ``test_run_context_app_config_reflects_yaml_edit``
exercises the worker-feeding path: it asserts that ``ctx.app_config``
follows a mid-flight ``config.yaml`` edit while
``ctx.run_events_config`` stays frozen to the startup snapshot the
event store was built from.

Refs: bytedance/deer-flow#3107 (BUG-001), bytedance/deer-flow#3131 review

backend/tests/test_gateway_config_freshness.py

@@ -111,6 +111,52 @@ def test_get_config_respects_test_set_app_config():
     assert client.get("/probe").json() == {"log_level": "warning"}
 
 
+def test_run_context_app_config_reflects_yaml_edit(tmp_path, monkeypatch):
+    """``RunContext.app_config`` must follow live `config.yaml` edits.
+
+    BUG-001 review feedback: the run-context that feeds worker / lead-agent
+    factories must observe the same mtime reload that `get_config()` does;
+    otherwise stale config slips back in through the run path even after the
+    request dependency is fixed.
+    """
+    from unittest.mock import MagicMock
+
+    from app.gateway.deps import get_run_context
+
+    config_file = tmp_path / "config.yaml"
+    _write_config_yaml(config_file, log_level="info")
+    monkeypatch.setenv("DEER_FLOW_CONFIG_PATH", str(config_file))
+
+    app = FastAPI()
+    # Sentinel values for the rest of the RunContext wiring — we only care
+    # about ``ctx.app_config`` for this assertion.
+    app.state.checkpointer = MagicMock()
+    app.state.store = MagicMock()
+    app.state.run_event_store = MagicMock()
+    app.state.run_events_config = {"frozen": "startup"}
+    app.state.thread_store = MagicMock()
+
+    @app.get("/run-ctx-log-level")
+    def probe(ctx=Depends(get_run_context)):
+        return {
+            "log_level": ctx.app_config.log_level,
+            "run_events_config": ctx.run_events_config,
+        }
+
+    client = TestClient(app)
+    first = client.get("/run-ctx-log-level").json()
+    assert first == {"log_level": "info", "run_events_config": {"frozen": "startup"}}
+
+    _write_config_yaml(config_file, log_level="debug")
+    future_mtime = config_file.stat().st_mtime + 5
+    os.utime(config_file, (future_mtime, future_mtime))
+
+    second = client.get("/run-ctx-log-level").json()
+    # app_config follows the edit; run_events_config stays frozen to the
+    # startup snapshot we wrote onto app.state above.
+    assert second == {"log_level": "debug", "run_events_config": {"frozen": "startup"}}
+
+
 @pytest.mark.parametrize(
     "exception",
     [