Fix Render port binding - preload_app=False, add early env debug

backend/app.py

@@ -1,6 +1,12 @@
 # Ensure typing constructs and models are available globally for FastAPI type annotation evaluation
+import os
+
+# Print env vars immediately - BEFORE any imports
+print(f"[app.py] EARLY - PORT={os.getenv('PORT')}, HOST={os.getenv('HOST')}", flush=True)
+
 import typing
 import builtins
+import builtins
 
 # Make common typing constructs available globally
 builtins.Optional = typing.Optional