Remove temporary debug-headers route

Confirmed fixed: /owner/_debug_headers showed scheme=https and all
three X-Forwarded-* headers present after the Waitress trusted_proxy
fix.

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

backoffice/app/app.py

@@ -84,11 +84,6 @@ def rows_json(rows):
     return [{k: jsonable(v) for k, v in r.items()} for r in rows]
 
 
-@app.get("/owner/_debug_headers")
-def _debug_headers():
-    return jsonify({"scheme": request.scheme, "headers": dict(request.headers)})
-
-
 @app.get("/healthz")
 def healthz():
     try: