Add frontend route /admin/libraries/:id/issues to display processing issues
management page for a specific library.
internal/router/frontend.go:
- Register GET /admin/libraries/:id/issues with admin middleware
- Fetch processing issue stats from database
- List processing issues for the library
- Convert database models to template types
- Render AdminProcessingIssues template with issues and stats
This provides the admin UI for viewing and managing processing errors that
occur during media scanning and import workflows.