Add UK education verification and security fixes

Features:
- Add UK institution recognition (170+ universities)
- Add diploma mill detection (100+ blacklisted institutions)
- Add education verification service with date plausibility checks
- Add local file storage option (no Azure required)
- Add default admin user seeding on startup
- Enhance Serilog logging with file output

Security fixes:
- Fix path traversal vulnerability in LocalFileStorageService
- Fix open redirect in login endpoint (use LocalRedirect)
- Fix password validation message (12 chars, not 6)
- Fix login to use HTTP POST endpoint (avoid Blazor cookie issues)

Code improvements:
- Add CancellationToken propagation to CV parser
- Add shared helpers (JsonDefaults, DateHelpers, ScoreThresholds)
- Add IUserContextService for user ID extraction
- Parallelized company verification in ProcessCVCheckJob
- Add 28 unit tests for education verification

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

src/TrueCV.Application/Helpers/DateHelpers.cs

@@ -0,0 +1,19 @@
+namespace TrueCV.Application.Helpers;
+
+public static class DateHelpers
+{
+    public static DateOnly? ParseDate(string? dateString)
+    {
+        if (string.IsNullOrWhiteSpace(dateString))
+        {
+            return null;
+        }
+
+        if (DateOnly.TryParse(dateString, out var date))
+        {
+            return date;
+        }
+
+        return null;
+    }
+}