TASK-001: Setup FastAPI project structure

- Fixed main.py to include all route routers (posts, users, comments, feed)
- Renamed app/models.py to app/schemas.py and split into proper schema modules
- Fixed schema imports in routes
- Updated app/models/__init__.py to properly export SQLAlchemy models
- Fixed database imports in route files
- App imports and runs correctly

app/models/__init__.py

@@ -1,4 +1,6 @@
 """Models package."""
+
+# SQLAlchemy models
 from app.models.user import User
 
 __all__ = ["User"]