Add controller logging and return readable errors

The controller ran silently and surfaced errors as opaque/empty bodies,
making it impossible to diagnose failures from either the UI or the server.
- Configure Python logging (timestamps, levels) and log job lifecycle events
  (created / dispatched / done / failed) plus startup and dispatch warnings.
- Add a catch-all error handler so unhandled exceptions return a JSON error
  with the exception type+message instead of an empty response.
- Remove the frontend submit bug that hand-set multipart Content-Type without
  a boundary (and built an opts object it never used); post the FormData
  directly so the browser sets the correct content type + boundary.

ctrl/app.py

@@ -1,5 +1,6 @@
 import datetime
 import json
+import logging
 import os
 import queue
 
@@ -10,6 +11,8 @@ import ctrl.settings as settings
 import ctrl.workers as wrk
 from ctrl.scheduler import make_scheduler
 
+log = logging.getLogger("ctrl")
+
 app = Flask(__name__, static_folder="static", static_url_path="")
 scheduler = make_scheduler()
 
@@ -75,6 +78,7 @@ def api_create_job():
         job_id = db.create_job("git", repo_url, os_req, cpu_req)
 
     scheduler.start()
+    log.info("Job %s created: type=%s os=%s cpu=%s", job_id, "upload" if "file" in request.files else "git", os_req, cpu_req)
     return jsonify({"id": job_id}), 201
 
 
@@ -144,9 +148,17 @@ def api_stream(job_id):
     return Response(gen(), mimetype="text/event-stream")
 
 
+@app.errorhandler(Exception)
+def _handle_error(err):
+    log.exception("Unhandled error serving %s %s", request.method, request.path)
+    return jsonify({"error": f"{type(err).__name__}: {err}"}), 500
+
+
 def main():
     _ensure_ready()
     scheduler.start()
+    _configure_logging()
+    log.info("Controller listening on 0.0.0.0:%s (%d workers configured)", settings.PORT, len(settings.WORKERS))
     if settings.DEBUG:
         app.run(host="0.0.0.0", port=settings.PORT, threaded=True)
         return
@@ -154,5 +166,16 @@ def main():
     serve(app, host="0.0.0.0", port=settings.PORT, threads=8)
 
 
+def _configure_logging():
+    level = logging.DEBUG if settings.DEBUG else logging.INFO
+    logging.basicConfig(
+        level=level,
+        format="%(asctime)s %(levelname)s %(name)s: %(message)s",
+        datefmt="%Y-%m-%d %H:%M:%S",
+    )
+    # waitress's own request access log to stderr by default; keep ours separate.
+    logging.getLogger("waitress").setLevel(logging.INFO)
+
+
 if __name__ == "__main__":
     main()