fixup! 📦 Build: Modernize Python packaging and Flask 3.x compatibility

🔧 App: Disable debug mode by default, use `FLASK_DEBUG` env var

Debug mode was hardcoded to `True`, which is unsafe for production.
Default to off and read from the `FLASK_DEBUG` environment variable
instead. Run with `FLASK_DEBUG=1` for local development.

Assisted-by: Claude Opus 4.6 (1M context)
Signed-off-by: Justin Wheeler <git@jwheel.org>

Author: justwheel

app.py

@@ -1,4 +1,6 @@
+import os
+
 from hflossk.site import app
 
 if __name__ == "__main__":
-    app.run(debug=True)
+    app.run(debug=os.environ.get("FLASK_DEBUG", "0") == "1")