chore: Add uv.lock and gitignore celerybeat-schedule files

- Updated uv.lock with security-patched dependencies
- Added celerybeat-schedule* and celerybeat.pid to .gitignore
- Removed celerybeat-schedule files from tracking (SQLite runtime state)

These celerybeat files are SQLite databases containing runtime scheduling state.
They should not be tracked because:
- They're binary files that change during celery beat execution
- They cause merge conflicts between developers
- Each environment regenerates them automatically on startup

.gitignore

@@ -30,6 +30,10 @@ db.sqlite3-journal
 /backend/staticfiles/
 /backend/media/
 
+# Celery Beat schedule database (runtime state, regenerated automatically)
+celerybeat-schedule*
+celerybeat.pid
+
 # UV
 .uv/
 backend/.uv/