Implement code review fixes and improvements

- Move admin credentials from hardcoded values to configuration
- Add rate limiting (5/min) to login endpoint for brute force protection
- Extract CleanJsonResponse to shared JsonResponseHelper class
- Add DateHelpers.MonthsBetween utility and consolidate date calculations
- Update PdfReportService to use ScoreThresholds constants
- Remove 5 unused shared components (EmploymentTable, FlagsList, etc.)
- Clean up unused CSS from MainLayout.razor.css
- Create IPdfReportService interface for better testability
- Add authentication requirement to Hangfire dashboard in development
- Seal EducationVerifierService class

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/TrueCV.Infrastructure/Services/EducationVerifierService.cs

@@ -4,7 +4,7 @@ using TrueCV.Application.Models;
 
 namespace TrueCV.Infrastructure.Services;
 
-public class EducationVerifierService : IEducationVerifierService
+public sealed class EducationVerifierService : IEducationVerifierService
 {
     private const int MinimumDegreeYears = 1;
     private const int MaximumDegreeYears = 8;