feat: enhance Stripstream configuration handling

- Introduced a new resolver function to streamline fetching Stripstream configuration from the database or environment variables.
- Updated various components and API routes to utilize the new configuration resolver, improving code maintainability and reducing direct database calls.
- Added optional environment variables for Stripstream URL and token in the .env.example file.
- Refactored image loading logic in the reader components to improve performance and error handling.

.env.example

@@ -5,4 +5,8 @@ MONGODB_URI=mongodb://admin:password@host.docker.internal:27017/stripstream?auth
 
 NEXTAUTH_SECRET=SECRET 
 #openssl rand -base64 32
-NEXTAUTH_URL=http://localhost:3000
+NEXTAUTH_URL=http://localhost:3000
+
+# Stripstream Librarian (optionnel : fallback si l'utilisateur n'a pas sauvegardé d'URL/token en base)
+# STRIPSTREAM_URL=https://librarian.example.com
+# STRIPSTREAM_TOKEN=stl_xxxx_xxxxxxxx