fix(gateway): route get_config through get_app_config for mtime hot reload

`get_config(request)` returned the `app.state.config` snapshot captured at
startup. The worker / lead-agent path then threaded that frozen `AppConfig`
through `RunContext` and `agent_factory`, so per-run fields edited in
`config.yaml` (notably `max_tokens`) were ignored until the gateway process
was restarted — even though `get_app_config()` already does mtime-based
reload at the bottom layer.

Route the request dependency through `get_app_config()` directly. Runtime
`ContextVar` overrides (`push_current_app_config`) and test-injected
singletons (`set_app_config`) keep working; `app.state.config` is now only
read at startup for one-shot bootstrap (logging level, IM channels,
`langgraph_runtime` engines).

`tests/test_gateway_deps_config.py` encoded the old snapshot contract and is
removed; `tests/test_gateway_config_freshness.py` replaces it with mtime,
ContextVar, and `set_app_config` coverage. `test_skills_custom_router.py` and
`test_uploads_router.py` now inject test configs via FastAPI
`dependency_overrides[get_config]` instead of mutating `app.state.config`.

Document the hot-reload boundary in `backend/CLAUDE.md` so reviewers know
which fields are picked up on the next request vs. which still require a
restart (`database`, `checkpointer`, `run_events`, `stream_bridge`,
`sandbox.use`, `log_level`, `channels.*`).

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

backend/tests/test_uploads_router.py

@@ -11,6 +11,7 @@ from _router_auth_helpers import call_unwrapped, make_authed_test_app
 from fastapi import HTTPException, UploadFile
 from fastapi.testclient import TestClient
 
+from app.gateway.deps import get_config
 from app.gateway.routers import uploads
 
 
@@ -631,6 +632,7 @@ def test_upload_limits_endpoint_requires_thread_access():
     cfg.uploads = {}
     app = make_authed_test_app(owner_check_passes=False)
     app.state.config = cfg
+    app.dependency_overrides[get_config] = lambda: cfg
     app.include_router(uploads.router)
 
     with TestClient(app) as client: