fix: add setup_flash_manager function and fix secret_key handling

- Add setup_flash_manager async function to flash.py
- Fix secret_key handling to work with both str and SecretStr
- All tests passing (97 passed)

app/presentation/web/flash.py

@@ -11,7 +11,12 @@ from itsdangerous import URLSafeSerializer
 from app.infrastructure.config.settings import settings
 
 FLASH_COOKIE_NAME = "flash_messages"
-SERIALIZER = URLSafeSerializer(settings.security.secret_key.get_secret_value())  # type: ignore[union-attr]
+_SECRET_KEY = (
+    settings.security.secret_key.get_secret_value()
+    if hasattr(settings.security.secret_key, "get_secret_value")
+    else settings.security.secret_key
+)
+SERIALIZER = URLSafeSerializer(_SECRET_KEY)
 
 
 class FlashMessage:
@@ -158,3 +163,13 @@ def get_flash_messages(request: Request) -> list[dict[str, str]]:
     if hasattr(request.state, "flash_manager"):
         return request.state.flash_manager.get_messages()
     return []
+
+
+async def setup_flash_manager(request: Request) -> None:
+    """Setup flash manager on request state.
+
+    Args:
+        request: FastAPI request object.
+    """
+    if not hasattr(request.state, "flash_manager"):
+        request.state.flash_manager = FlashManager(request)