Add manual backup trigger endpoint

POST /api/v1/system/auto-backup/trigger creates a backup on demand
and returns the created file info (filename, size, timestamp).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/api/routes/system.py

@@ -489,6 +489,16 @@ async def update_auto_backup_settings(
     )
 
 
+@router.post("/api/v1/system/auto-backup/trigger", tags=["System"])
+async def trigger_backup(
+    _: AuthRequired,
+    engine: AutoBackupEngine = Depends(get_auto_backup_engine),
+):
+    """Manually trigger a backup now."""
+    backup = await engine.trigger_backup()
+    return {"status": "ok", "backup": backup}
+
+
 @router.get(
     "/api/v1/system/backups",
     response_model=BackupListResponse,