perf: unblock event loop for segment_efforts scan

Extract the synchronous segment-file scan into a plain function and
dispatch it via asyncio.to_thread so it runs in a thread pool instead
of blocking the event loop during concurrent fetches.

bincio/serve/routers/activities.py

@@ -351,13 +351,16 @@ async def activity_segment_efforts(
     bincio_session: str | None = Cookie(default=None),
 ) -> JSONResponse:
     """Return segment efforts that belong to a specific activity for the logged-in user."""
+    import asyncio
     from bincio.segments import store as _seg_store
     user = deps._require_user(bincio_session)
     dd = deps._get_data_dir()
+
+    def _collect() -> list[dict]:
         efforts_dir = dd / user.handle / "segment_efforts"
-    result = []
+        result: list[dict] = []
-    if efforts_dir.exists():
+        if not efforts_dir.exists():
-        import json as _json
+            return result
         for ef_file in sorted(efforts_dir.glob("*.json")):
             seg_id = ef_file.stem
             all_efforts = _seg_store.load_efforts(dd, user.handle, seg_id)
@@ -377,4 +380,6 @@ async def activity_segment_efforts(
                     "pr_elapsed_s":         pr_elapsed,
                     "started_at":           _seg_store._iso(eff.started_at),
                 })
-    return JSONResponse(result)
+        return result
+
+    return JSONResponse(await asyncio.to_thread(_collect))