feat(newsletter): Add subscription confirmation email functionality

- Implemented `send_subscription_confirmation` function to send a confirmation email upon subscription.
- Added a default HTML template for the confirmation email in settings.
- Updated the newsletter management page to handle subscription and unsubscription actions.
- Created new templates for embedding contact and newsletter forms.
- Added styles for the new templates and unified CSS styles across the application.
- Updated tests to reflect changes in the newsletter management API endpoints.

server/routes/__init__.py

@@ -3,7 +3,7 @@ from __future__ import annotations
 
 from flask import Flask
 
-from . import admin, auth, contact, monitoring, newsletter
+from . import admin, auth, contact, embed, monitoring, newsletter
 
 
 def register_blueprints(app: Flask) -> None:
@@ -13,3 +13,4 @@ def register_blueprints(app: Flask) -> None:
     app.register_blueprint(monitoring.bp)
     app.register_blueprint(auth.bp)
     app.register_blueprint(admin.bp)
+    app.register_blueprint(embed.bp)