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/dev.py

@@ -66,8 +66,10 @@ def _user_dirs(data: Path) -> list[Path]:
 
 def _merge_all_users(data: Path) -> None:
     from bincio.render.cli import _merge_edits, _write_root_manifest
+    from bincio.render.merge import write_combined_feed
     _merge_edits(data)
     _write_root_manifest(data)
+    write_combined_feed(data)
 
 
 def _local_ip() -> str:
@@ -148,8 +150,9 @@ def _watch_data(data: Path) -> None:
         for user_dir in affected:
             handle = user_dir.name
             try:
-                from bincio.render.merge import merge_all
+                from bincio.render.merge import merge_all, write_combined_feed
                 merge_all(user_dir)
+                write_combined_feed(data)
                 console.print(f"  [dim]↺  {handle}: merged[/dim]")
             except Exception as exc:
                 console.print(f"  [yellow]⚠  {handle}: merge failed — {exc}[/yellow]")