Do not trust empty backups when sources are configured

Restoring a backup with no data while sources are configured now sets the
db_recreated.json marker (503 until data is collected), and the backup job
skips copying an empty database in that state. Adds finding 11 to the review.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

collector_daemon.py

@@ -28,6 +28,9 @@ def run_backup():
     keep = max(1, int(load_full_config().get("backup_keep", cc.DEFAULT_BACKUP_KEEP)))
     with db.session() as conn:
         path = db.backup_database(conn, datetime.datetime.now(), keep)
+    if path is None:
+        logger.warning("Database backup skipped: no data yet although sources are configured")
+        return
     logger.info("Database backup written to %s (keeping the last %d)", path, keep)