feat(data-source): expose custom source timeout & request param names in UI

- backend: pass timeout through DatasetConfigIn / _config_to_dict / _sanitize_dataset
  (previously a hand-set YAML timeout was silently wiped on UI save, resetting to 30s)
- frontend: add per-dataset timeout input; collapsible 请求参数字段映射 with
  symbols/start/end_param (non-realtime) + asset_type/freq_param (minute);
  rename 字段映射 -> 响应参数字段映射; chevron at title end; realtime empty-state hint
- test: timeout config round-trip (custom value persists, default 30 not emitted)
- docs: document timeout in custom-data-source.md

backend/app/api/settings.py

@@ -373,6 +373,7 @@ class DatasetConfigIn(BaseModel):
     end_param: str = "end_time"
     asset_type_param: str | None = None
     freq_param: str | None = None
+    timeout: float | None = None
 
 
 class AuthConfigIn(BaseModel):