Fix disappearing artifacts and guard against empty builds

The agent's background build threads called jsonify()/send_file() with no
Flask app context, so after a successful build they crashed with
'Working outside of application context', which triggered the error
handler that rmtree'd the whole build dir before the controller could
download the artifact. Wrap both runners in app.app_context().

Also guard all three layers against empty results: the agent refuses to
report success or serve a zip when dist/ has no files, and the scheduler
flags a downloaded empty zip as a failure rather than marking the job
done.

ctrl/scheduler.py

@@ -3,6 +3,7 @@ import logging
 import os
 import threading
 import time
+import zipfile
 
 import requests
 
@@ -225,6 +226,12 @@ class Scheduler:
             for chunk in resp.iter_content(chunk_size=8192):
                 f.write(chunk)
 
+        with zipfile.ZipFile(dest) as zf:
+            files = [n for n in zf.namelist() if not n.endswith("/")]
+        if not files:
+            os.remove(dest)
+            raise RuntimeError(f"Worker {worker['url']} returned an empty artifact zip for {worker_job_id}")
+
         db.update_job(job_id, artifacts=[fname])
 
     # ---- logging / status -------------------------------------------------