app_module_for_backend: wait for _compile in prod mode

Prod mode uses separate worker processes that fork from the main process.

If the app is not fully compiled when the fork occurs, any further changes to
the app (like mounting the _upload endpoint) will not be reflected in the
workers. This is not a performance hit because compile is skipped anyway
for backend processes and hot reload is not in the picture for prod mode.

reflex/app_module_for_backend.py

@@ -4,6 +4,7 @@ Only the app attribute is explicitly exposed.
 from concurrent.futures import ThreadPoolExecutor
 
 from reflex import constants
+from reflex.utils.exec import is_prod_mode
 from reflex.utils.prerequisites import get_app, get_compiled_app
 
 if "app" != constants.CompileVars.APP:
@@ -11,14 +12,20 @@ if "app" != constants.CompileVars.APP:
 
 app_module = get_app(reload=False)
 app = getattr(app_module, constants.CompileVars.APP)
-# Force background compile errors to print eagerly
-ThreadPoolExecutor(max_workers=1).submit(app.compile_).add_done_callback(
+compile_future = ThreadPoolExecutor(max_workers=1).submit(app.compile_)
+compile_future.add_done_callback(
+    # Force background compile errors to print eagerly
     lambda f: f.result()
 )
+# Wait for the compile to finish in prod mode to ensure all optional endpoints are mounted.
+if is_prod_mode():
+    compile_future.result()
 
 # ensure only "app" is exposed.
 del app_module
+del compile_future
 del get_app
 del get_compiled_app
+del is_prod_mode
 del constants
 del ThreadPoolExecutor