fix(upload): prevent false 422s and EMFILE crash during bulk uploads

Four related issues made uploading 271+ activities unreliable:

1. merge_all/write_combined_feed were inside the extraction try/except —
   any merge race returned 422 even though the file was on disk, causing
   the mobile app to permanently mark the upload as failed.  Fixed by
   moving them to a separate best-effort try/except after the extraction
   block.  Switch to merge_one (single-activity symlink) instead of
   merge_all (full rebuild) so each upload is O(1) FS ops, not O(N).

2. The dev watcher fired merge_all for every activity .json write AND the
   upload endpoint also ran merge_all — O(N²) symlink operations during
   bulk uploads.  Watcher now skips activities/*.json changes (upload
   endpoint handles those directly).

3. Vite/Chokidar followed the public/data symlink and opened a handle per
   activity file; constant merge rebuilds exhausted file descriptors and
   crashed the Astro dev server.  Fixed with watch.ignored on public/data.

4. _write_year_shards and write_combined_feed used f.unlink() without
   missing_ok=True — concurrent callers racing the same file threw
   FileNotFoundError which propagated as a false extraction failure.

bincio/serve/server.py

@@ -596,9 +596,12 @@ async def upload_bas_activity(
 
     _upsert_index_summary(user_dir, activity_id, activity, geojson_body)
 
-    from bincio.render.merge import merge_all, write_combined_feed
-    merge_all(user_dir)
-    write_combined_feed(_get_data_dir())
+    try:
+        from bincio.render.merge import merge_one, write_combined_feed
+        merge_one(user_dir, activity_id)
+        write_combined_feed(_get_data_dir())
+    except Exception as exc:
+        log.warning("upload/bas[%s]: merge/feed failed (non-fatal): %s", user.handle, exc)
 
     log.info("upload/bas[%s]: imported %s", user.handle, activity_id)
     return JSONResponse({"ok": True, "id": activity_id, "status": "imported"})
@@ -688,10 +691,6 @@ async def upload_raw_activity(
 
         _upsert_index_summary(user_dir, act_id, detail, geojson)
 
-        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)
         raise HTTPException(422, f"Could not extract activity: {exc}") from exc
@@ -699,6 +698,16 @@ async def upload_raw_activity(
         tmp_in.unlink(missing_ok=True)
         shutil.rmtree(tmp_out, ignore_errors=True)
 
+    # Merge and update feed — best effort; a race or transient FS error here must
+    # not turn a successful extraction into a 422 (the file is on disk; the mobile
+    # would retry indefinitely and the activity would never be marked synced).
+    try:
+        from bincio.render.merge import merge_one, write_combined_feed
+        merge_one(user_dir, act_id)
+        write_combined_feed(_get_data_dir())
+    except Exception as exc:
+        log.warning("upload/raw[%s]: merge/feed failed (non-fatal): %s", user.handle, exc)
+
     log.info("upload/raw[%s]: imported %s", user.handle, act_id)
     return JSONResponse({
         "ok": True,