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/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