### Description
This PR addresses orphaned user-keyed tables during user deletion by
adding the missing models to the `db.DeleteBeans` call in
`services/user/delete.go`:
- `auth_model.TwoFactor` (TOTP secrets)
- `auth_model.WebAuthnCredential` (WebAuthn keys)
- `activities_model.Notification` (Notifications)
- `issues_model.IssueWatch` (Issue watches)
Additionally, it adds corresponding unit test coverage in
`services/user/user_test.go` to assert that these records are cleaned up
successfully.
### Related Issues
Fixes#38510
Signed-off-by: pranav718 <raypranav718@gmail.com>
c6791c3c58