show startup traceback in fallback mode

main.py

@@ -17,10 +17,11 @@ try:
     app = create_app()
 
 except Exception as e:
-    print("✗ Fallback activated due to error:")
     import traceback
 
-    traceback.print_exc()
+    _err_text = traceback.format_exc()
+    print("✗ Fallback activated due to error:")
+    print(_err_text)
 
     from flask import Flask
 
@@ -29,7 +30,12 @@ except Exception as e:
 
     @app.route("/")
     def hello():
-        return "Hello World! (fallback mode)"
+        # Surface the real startup error in the browser to aid debugging.
+        return (
+            "<h2>Startup failed (fallback mode)</h2><pre>" + _err_text + "</pre>",
+            500,
+            {"Content-Type": "text/html; charset=utf-8"},
+        )
 
 
     @app.route("/healthz")