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/Helpers/JsonResponseHelper.cs

@@ -0,0 +1,32 @@
+namespace TrueCV.Infrastructure.Helpers;
+
+/// <summary>
+/// Helper methods for processing AI/LLM JSON responses.
+/// </summary>
+public static class JsonResponseHelper
+{
+    /// <summary>
+    /// Cleans a JSON response by removing markdown code block formatting.
+    /// </summary>
+    public static string CleanJsonResponse(string response)
+    {
+        var trimmed = response.Trim();
+
+        // Remove markdown code blocks
+        if (trimmed.StartsWith("```json", StringComparison.OrdinalIgnoreCase))
+        {
+            trimmed = trimmed[7..];
+        }
+        else if (trimmed.StartsWith("```"))
+        {
+            trimmed = trimmed[3..];
+        }
+
+        if (trimmed.EndsWith("```"))
+        {
+            trimmed = trimmed[..^3];
+        }
+
+        return trimmed.Trim();
+    }
+}