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/models/user.py

@@ -16,6 +16,8 @@ class User(BaseModel):
     pin: str = ''
     pin_setup_code: str = ''
     pin_setup_code_created: str | None = None
+    marked_for_deletion: bool = False
+    marked_for_deletion_at: str | None = None
 
     @classmethod
     def from_dict(cls, d: dict):
@@ -33,6 +35,8 @@ class User(BaseModel):
             pin=d.get('pin', ''),
             pin_setup_code=d.get('pin_setup_code', ''),
             pin_setup_code_created=d.get('pin_setup_code_created'),
+            marked_for_deletion=d.get('marked_for_deletion', False),
+            marked_for_deletion_at=d.get('marked_for_deletion_at'),
             id=d.get('id'),
             created_at=d.get('created_at'),
             updated_at=d.get('updated_at')
@@ -54,6 +58,8 @@ class User(BaseModel):
             'image_id': self.image_id,
             'pin': self.pin,
             'pin_setup_code': self.pin_setup_code,
-            'pin_setup_code_created': self.pin_setup_code_created
+            'pin_setup_code_created': self.pin_setup_code_created,
+            'marked_for_deletion': self.marked_for_deletion,
+            'marked_for_deletion_at': self.marked_for_deletion_at
         })
         return base