Add activity log for important events

Implement an activity log that records key events such as application creation, status changes, notes created, and tasks completed. Provide endpoints to fetch activity per application and a global activity feed for the user.

Look for existing models like Application, Note, and Task to understand the events to log. Check the backend structure for where to add an Activity model and its service. The endpoints need to be added to the API router, likely in a file like `src/routes/activity.ts`. Start by defining the activity schema and the events to capture, then implement the logging in relevant service methods. 'Done' means the new endpoints return activity data for an application and a user-wide feed.

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