Fix: Assign Internal API User to Internal Team and Eager-Load User’s Team Association (#3698)

# Description of Changes

- **What was changed**  
- Added logic in `InitialSecuritySetup` to assign the
`INTERNAL_API_USER` to a dedicated “internal” team both during initial
creation and on subsequent startups.
- Enhanced `assignUsersToDefaultTeamIfMissing()` to route the internal
API user to the `internalTeam`, while all other users go to the default
team.
- Switched the JPA mapping of `User.team` from `LAZY` to `EAGER` fetch
to ensure the team association is always loaded with the user.
- Introduced a new `UserService.changeUserTeam(User, Team)` method to
handle moving an existing user to a different team and persist the
change.
- Imported `java.util.Optional` to safely handle lookups of the internal
API user.

- **Why the change was made**  
- To guarantee that the special internal API user is always part of the
“internal” team and never left on the default team, preventing
permission and routing issues.
- Eagerly loading the `team` association avoids lazy-init exceptions in
contexts where the user’s team is needed immediately (e.g., security
checks).

---

## Checklist

### General

- [x] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [x] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/HowToAddNewLanguage.md)
(if applicable)
- [x] I have performed a self-review of my own code
- [x] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md#6-testing)
for more details.