fix(theming): read cachebuster as int in manifest endpoint

[solracsf]

• Resolves: [Bug]: conflict with value type from database AppConfigTypeConflictException conflict with value type from database #60731

Summary

The theming/cachebuster app config value is declared as ValueType::INT in the config lexicon, stored as INT, and written/read as INT everywhere (ImageManager, ThemingDefaults). ThemingController::getManifest() was the
only place reading it via getAppValueString(), which requests VALUE_STRING and conflicts with the stored VALUE_INT, throwing AppConfigTypeConflictException on every manifest request

Regression introduced in #60198

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI

[solracsf]

/backport to stable34

[solracsf]

/backport to stable33

[solracsf]

/backport to stable32

[backportbot]

The backport to stable33 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable33
git pull origin stable33

# Create the new backport branch
git checkout -b backport/61007/stable33

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick 17bbc56d

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/61007/stable33

Error: Failed to check for changes with origin/stable33: No changes found in backport branch

---

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.

[solracsf]

/backport to stable33

[backportbot]

The backport to stable33 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable33
git pull origin stable33

# Create the new backport branch
git checkout -b backport/61007/stable33

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick 17bbc56d

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/61007/stable33

Error: Failed to check for changes with origin/stable33: No changes found in backport branch

---

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.