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/app/gateway/app.py

@@ -161,10 +161,16 @@ async def _migrate_orphaned_threads(store, admin_user_id: str) -> int:
 async def lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
     """Application lifespan handler."""
 
-    # Load config and check necessary environment variables at startup
+    # Load config and check necessary environment variables at startup.
+    # `startup_config` is a local snapshot used only for one-shot bootstrap
+    # work (logging level, langgraph_runtime engines, channels). Request-time
+    # config resolution always routes through `get_app_config()` in
+    # `app/gateway/deps.py::get_config()` so `config.yaml` edits become
+    # visible without a process restart. We deliberately do NOT cache this
+    # snapshot on `app.state` to keep that contract enforceable.
     try:
-        app.state.config = get_app_config()
-        apply_logging_level(app.state.config.log_level)
+        startup_config = get_app_config()
+        apply_logging_level(startup_config.log_level)
         logger.info("Configuration loaded successfully")
     except Exception as e:
         error_msg = f"Failed to load configuration during gateway startup: {e}"
@@ -174,7 +180,7 @@ async def lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
     logger.info(f"Starting API Gateway on {config.host}:{config.port}")
 
     # Initialize LangGraph runtime components (StreamBridge, RunManager, checkpointer, store)
-    async with langgraph_runtime(app):
+    async with langgraph_runtime(app, startup_config):
         logger.info("LangGraph runtime initialised")
 
         # Check admin bootstrap state and migrate orphan threads after admin exists.
@@ -185,7 +191,7 @@ async def lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
         try:
             from app.channels.service import start_channel_service
 
-            channel_service = await start_channel_service(app.state.config)
+            channel_service = await start_channel_service(startup_config)
             logger.info("Channel service started: %s", channel_service.get_status())
         except Exception:
             logger.exception("No IM channels configured or channel service failed to start")