feat: Implement account deletion (mark for removal) feature

- Added `marked_for_deletion` and `marked_for_deletion_at` fields to User model (Python and TypeScript) with serialization updates
- Created POST /api/user/mark-for-deletion endpoint with JWT auth, error handling, and SSE event trigger
- Blocked login and password reset for marked users; added new error codes ACCOUNT_MARKED_FOR_DELETION and ALREADY_MARKED
- Updated UserProfile.vue with "Delete My Account" button, confirmation modal (email input), loading state, success/error modals, and sign-out/redirect logic
- Synced error codes and model fields between backend and frontend
- Added and updated backend and frontend tests to cover all flows and edge cases
- All Acceptance Criteria from the spec are complete and verified

backend/events/types/event_types.py

@@ -13,3 +13,5 @@ class EventType(Enum):
     CHILD_REWARD_REQUEST = "child_reward_request"
 
     CHILD_MODIFIED = "child_modified"
+
+    USER_MARKED_FOR_DELETION = "user_marked_for_deletion"

backend/events/types/user_modified.py

@@ -0,0 +1,21 @@
+from events.types.payload import Payload
+
+
+class UserModified(Payload):
+    OPERATION_ADD = "ADD"
+    OPERATION_EDIT = "EDIT"
+    OPERATION_DELETE = "DELETE"
+    def __init__(self, user_id: str, operation: str):
+        super().__init__({
+            'user_id': user_id,
+            'operation': operation,
+        })
+
+    @property
+    def user_id(self) -> str:
+        return self.get("user_id")
+
+    @property
+    def operation(self) -> str:
+        return self.get("operation")
+