Automated Email Notification System

[Vanessa-Chung-1372]

Summary:
This PR introduces the initial implementation of the Email Notification System for Keep.id. It enables automated email alerts for specific user activities using SendGrid.

Key Features:

1. Email Templates: HTML templates are separated from code for maintainability. Each activity type has a corresponding email layout for consistent branding and messaging.
2. SendGrid Integration: Implemented via a SendgridService class for sending transactional emails.
3. Composite Service Workflow:

• Calls both ActivityService.save() and EmailService.send() when handling activity.
• Supports future extension (e.g., logging, analytics, or message queueing).

4. Activity-Driven Notification:

• The system checks if an activity type is worth notifying before sending.
• Modular handleCreateClientActivity and similar methods handle specific cases.

5. Extensibility:

• Easy to add new templates and activity types.
• Prepared for future enhancement with message queues if needed.

Testing:

• Integration test for triggering notification on CreateClientActivity. (EmailIntTest)
• If this is in the right direction, we could figure out the activities types that worth sending email and decide the email templates.

<img width="894" height="648" alt="截圖 2025-07-14 下午8 09 35" src="https://github.com/user-attachments/assets/a50f3758-ffa6-4a11-ab08-93d4dc1bc844" /