feat(serve): debounced site rebuild — burst uploads trigger one build, not N

Replace per-upload Astro build threads with a single background worker
(_site_rebuild_worker) that waits on an event, sleeps 60 s to let upload
bursts settle, then runs one full build + rsync. 271 concurrent uploads now
produce one build instead of 271 serialised builds, eliminating the OOM kill.
--webroot is re-enabled; merge-only path still runs immediately per upload.

Also: date filter row added to ActivityFeed.svelte (sport + date presets
with dynamic year pills); deploy/vps gitignored for VPS config backups.

.gitignore

@@ -32,6 +32,9 @@ bincio_data/
 .env
 extract_config.yaml
 
+# VPS config backup (contains secrets, keep local only)
+deploy/vps/
+
 # Local working / scratch files
 advice.md
 issues.md