refactor(api): extract JobApplications DTOs and helpers, fix N+1 aggregation

- Move inline DTOs to JobApplicationDtos.cs, pure static helpers to JobApplicationHelpers.cs
- GetStats aggregates server-side (COUNT/GROUP BY) instead of loading the full table
- Cache RuleSettings via IMemoryCache, keyed per-user (RulesEngine.GetSettings falls back
  to per-user UserRuleSettings overrides, so a single global cache key would leak settings
  across users)
- Add missing AsNoTracking() to read-only GET endpoints (GetAll, GetById, GetBoard,
  GetReminders, GetStatusSuggestion, GetMatchScore, GetCandidateFit, GetFocusPlan,
  GetInterviewPrep, GetReadiness)

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

JobTrackerApi.Tests/JobApplicationsControllerTests.cs

@@ -9,7 +9,7 @@ public sealed class JobApplicationsControllerTests
     [Fact]
     public void Application_package_record_exposes_expected_fields()
     {
-        var type = typeof(JobApplicationsController).GetNestedType("GenerateApplicationPackageDto", BindingFlags.Public | BindingFlags.NonPublic);
+        var type = typeof(GenerateApplicationPackageDto);
         Assert.NotNull(type);
 
         var props = type!.GetProperties(BindingFlags.Public | BindingFlags.Instance).Select(x => x.Name).ToHashSet();
@@ -23,7 +23,7 @@ public sealed class JobApplicationsControllerTests
     [Fact]
     public void Save_application_drafts_request_supports_cover_letter_and_notes()
     {
-        var type = typeof(JobApplicationsController).GetNestedType("SaveApplicationDraftsRequest", BindingFlags.Public | BindingFlags.NonPublic);
+        var type = typeof(SaveApplicationDraftsRequest);
         Assert.NotNull(type);
 
         var ctor = type!.GetConstructors().Single();