refactor(config): eliminate global mutable state, wire DeerFlowContext into runtime

- Freeze all config models (AppConfig + 15 sub-configs) with frozen=True
- Purify from_file() — remove 9 load_*_from_dict() side-effect calls
- Replace mtime/reload/push/pop machinery with single ContextVar + init_app_config()
- Delete 10 sub-module globals and their getters/setters/loaders
- Migrate 50+ consumers from get_*_config() to get_app_config().xxx

- Expand DeerFlowContext: app_config + thread_id + agent_name (frozen dataclass)
- Wire into Gateway runtime (worker.py) and DeerFlowClient via context= parameter
- Remove sandbox_id from runtime.context — flows through ThreadState.sandbox only
- Middleware/tools access runtime.context directly via Runtime[DeerFlowContext] generic
- resolve_context() retained at server entry points for LangGraph Server fallback

backend/app/channels/service.py

@@ -67,9 +67,9 @@ class ChannelService:
     @classmethod
     def from_app_config(cls) -> ChannelService:
         """Create a ChannelService from the application config."""
-        from deerflow.config.app_config import get_app_config
+        from deerflow.config.app_config import AppConfig
 
-        config = get_app_config()
+        config = AppConfig.current()
         channels_config = {}
         # extra fields are allowed by AppConfig (extra="allow")
         extra = config.model_extra or {}