feat: bind to 0.0.0.0 by default, allow HOST/PORT override

Makes the development server accessible from the local network without
editing code. HOST and PORT environment variables can still be used to
restrict binding.

run.py

@@ -1,6 +1,10 @@
+import os
+
 from app import create_app
 
 app = create_app()
 
 if __name__ == "__main__":
-    app.run(host="127.0.0.1", port=5000, debug=False)
+    host = os.environ.get("HOST", "0.0.0.0")
+    port = int(os.environ.get("PORT", "5000"))
+    app.run(host=host, port=port, debug=False)