Fix SSE route dependency and align architecture docs

backend/app/__init__.py

@@ -1,3 +1,14 @@
-from .main import app
+"""Initialize the app package."""
+# Keep package boundaries explicit so backend imports stay predictable.
 
-__all__ = ["app"]
+
+__all__ = ["app"]
+
+
+def __getattr__(name: str):
+    """Handle getattr for this module."""
+    if name == "app":
+        from .main import app
+
+        return app
+    raise AttributeError(name)