diff --git a/bincio/dev.py b/bincio/dev.py
index 8e7f6df..06e2ef4 100644
--- a/bincio/dev.py
+++ b/bincio/dev.py
@@ -141,6 +141,11 @@ def _watch_data(data: Path) -> None:
                 continue
             for prefix, user_dir in prefix_to_user.items():
                 if path.startswith(prefix):
+                    # Skip new-activity .json writes in activities/ — the upload
+                    # endpoint calls merge_one inline, so firing merge_all here
+                    # too would cause O(N²) full rebuilds during bulk uploads.
+                    if path.startswith(str(user_dir / "activities")) and path.endswith(".json"):
+                        break
                     affected.add(user_dir)
                     break
 
diff --git a/bincio/render/merge.py b/bincio/render/merge.py
index fc6c5da..264b6a7 100644
--- a/bincio/render/merge.py
+++ b/bincio/render/merge.py
@@ -98,6 +98,11 @@ def merge_one(data_dir: Path, activity_id: str) -> None:
 
     Use merge_all() for bulk operations (first run, Strava sync, etc.).
     """
+    with _merge_lock(data_dir):
+        _merge_one_locked(data_dir, activity_id)
+
+
+def _merge_one_locked(data_dir: Path, activity_id: str) -> None:
     edits_dir  = data_dir / "edits"
     acts_dir   = data_dir / "activities"
     merged_dir = data_dir / "_merged"
@@ -311,7 +316,7 @@ def _write_year_shards(merged_dir: Path, activities: list[dict], index_meta: dic
 
     # Remove stale year shard files from previous runs
     for f in merged_dir.glob("index-*.json"):
-        f.unlink()
+        f.unlink(missing_ok=True)
 
     by_year: dict[str, list[dict]] = defaultdict(list)
     for a in activities:
@@ -398,7 +403,7 @@ def write_combined_feed(data_dir: Path) -> int:
 
     # Remove stale feed pages
     for f in data_dir.glob("feed*.json"):
-        f.unlink()
+        f.unlink(missing_ok=True)
 
     if not all_activities:
         return 0
diff --git a/bincio/serve/server.py b/bincio/serve/server.py
index 920004c..8d8f474 100644
--- a/bincio/serve/server.py
+++ b/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,
diff --git a/site/astro.config.mjs b/site/astro.config.mjs
index 95e75cd..b18a482 100644
--- a/site/astro.config.mjs
+++ b/site/astro.config.mjs
@@ -39,6 +39,12 @@ export default defineConfig({
     // Proxy /api/* to bincio serve/edit so cookies work same-origin in dev.
     // In production nginx handles this — same pattern, no code change needed.
     server: {
+      watch: {
+        // public/data is a symlink to the live data dir; Chokidar follows it and
+        // opens a handle per file, which causes EMFILE during bulk uploads as
+        // activity count grows. Data files don't need HMR — exclude them.
+        ignored: ['**/public/data/**'],
+      },
       proxy: {
         // Both /api/upload and /api/upload/strava-zip return SSE streams in response
         // to POST requests. Vite's default proxy buffers the full body before forwarding,