Add account deletion scheduler and comprehensive tests

- Implemented account deletion scheduler in `account_deletion_scheduler.py` to manage user deletions based on a defined threshold.
- Added logging for deletion processes, including success and error messages.
- Created tests for deletion logic, including edge cases, retry logic, and integration tests to ensure complete deletion workflows.
- Ensured that deletion attempts are tracked and that users are marked for manual intervention after exceeding maximum attempts.
- Implemented functionality to check for interrupted deletions on application startup and retry them.

.github/copilot-instructions.md

@@ -4,6 +4,7 @@
 
 - **Stack**: Flask (Python, backend) + Vue 3 (TypeScript, frontend) + TinyDB (JSON, thread-safe, see `db/`).
 - **API**: RESTful endpoints in `api/`, grouped by entity (child, reward, task, user, image, etc). Each API file maps to a business domain.
+- **Nginx Proxy**: Frontend nginx proxies `/api/*` to backend, stripping the `/api` prefix. Backend endpoints should NOT include `/api` in their route definitions. Example: Backend defines `@app.route('/user')`, frontend calls `/api/user`.
 - **Models**: Maintain strict 1:1 mapping between Python `@dataclass`es (`backend/models/`) and TypeScript interfaces (`frontend/vue-app/src/common/models.ts`).
 - **Database**: Use TinyDB with `from_dict()`/`to_dict()` for serialization. All logic should operate on model instances, not raw dicts.
 - **Events**: Real-time updates via Server-Sent Events (SSE). Every mutation (add/edit/delete/trigger) must call `send_event_for_current_user` (see `backend/events/`).
@@ -19,7 +20,7 @@
 - **Code Style**:
   1. Follow PEP 8 for Python, and standard TypeScript conventions.
   2. Use type annotations everywhere in Python.
-  3. Place python changes after imports. Place all imports at the top of the file.
+  3. Place all imports at the top of the file.
   4. Vue files should specifically place `<template>`, `<script>`, then `<style>` in that order. Make sure to put ts code in `<script>` only.
 
 ## 🚦 Frontend Logic & Event Bus