feat(fish_api): add FishAction folder watch via Pydantic settings

Add ACTION_WATCH_* settings, background watch loop in FastAPI lifespan,
and shared action_watch service that updates app_state. Add fish-action-watch
CLI and optional FishAction watch_folder_predict.py for standalone use.

Made-with: Cursor

fish_api/app/action_watch_cli.py

@@ -0,0 +1,24 @@
+"""独立进程：仅根据 .env / 环境变量中的 action_watch_dir 跑目录监控（不启动 FastAPI）。"""
+
+from __future__ import annotations
+
+import asyncio
+import sys
+
+from app.services.action_watch import run_action_watch_loop
+from app.settings import get_settings
+
+
+def main() -> None:
+    s = get_settings()
+    if s.action_watch_dir is None:
+        print(
+            "未配置监控目录：请在 .env 中设置 ACTION_WATCH_DIR=/你的/mp4/目录",
+            file=sys.stderr,
+        )
+        raise SystemExit(1)
+    asyncio.run(run_action_watch_loop(s))
+
+
+if __name__ == "__main__":
+    main()