fix(feed): update feed.json after every upload so browser sees new activities

merge_all(user_dir) updates the per-user _merged/ shard but the home page
loads feed.json first via loadCombinedFeed. write_combined_feed was only
called by the CLI render command, not by the API upload endpoints or the
dev watcher, leaving feed.json permanently stale after any runtime upload.

Add write_combined_feed(_get_data_dir()) after every merge_all call in
/api/upload/bas, /api/upload/raw, the dev.py file watcher, and dev startup.

bincio/serve/server.py

@@ -532,8 +532,9 @@ async def upload_bas_activity(
         if not gj_path.exists():
             gj_path.write_text(json.dumps(body["geojson"], ensure_ascii=False), encoding="utf-8")
 
-    from bincio.render.merge import merge_all
+    from bincio.render.merge import merge_all, write_combined_feed
     merge_all(user_dir)
+    write_combined_feed(_get_data_dir())
 
     log.info("upload/bas[%s]: imported %s", user.handle, activity_id)
     return JSONResponse({"ok": True, "id": activity_id, "status": "imported"})
@@ -621,8 +622,9 @@ async def upload_raw_activity(
         if geojson and not (acts_dir / f"{act_id}.geojson").exists():
             (acts_dir / f"{act_id}.geojson").write_text(json.dumps(geojson), encoding="utf-8")
 
-        from bincio.render.merge import merge_all
+        from bincio.render.merge import merge_all, write_combined_feed
         merge_all(user_dir)
+        write_combined_feed(_get_data_dir())
 
     except Exception as exc:
         log.warning("upload/raw[%s]: extraction failed: %s", user.handle, exc)