feat: admin maintenance banner

[Ugooweb]

Closes #622

This PR introduces the /api/admin/maintenance/banner endpoint which allows administrators to set and update the system-wide maintenance banner. The existing implementation for the banner was successfully hooked up and mounted onto the admin router.

Changes

• Mounted createMaintenanceBannerRouter to /maintenance/banner within src/routes/admin.ts.
• Validated inputs via the underlying route implementation (ensures message is non-empty, and isActive is a boolean).
• Included structured audit logging for banner updates.

Testing

• Tests are included for the /api/admin/maintenance/banner route in banner.test.ts ensuring success behavior.
• Validated edge cases where the input is missing or malformed (e.g., returns 400 BadRequest if message is empty or if isActive is a string instead of boolean).
• Run the repository's standard test suite to ensure existing components and endpoints remain unaffected.

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[greatest0fallt1me]

Great work, this looks good. Thanks for the contribution!