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/JobApplicationsFollowUpDraftTests.cs

@@ -92,7 +92,7 @@ public sealed class JobApplicationsFollowUpDraftTests
         var result = await controller.GetFollowUpDraft(job.Id, "waiting-update", null, CancellationToken.None);
 
         var ok = Assert.IsType<OkObjectResult>(result.Result);
-        var payload = Assert.IsType<JobApplicationsController.FollowUpDraftDto>(ok.Value);
+        var payload = Assert.IsType<FollowUpDraftDto>(ok.Value);
 
         Assert.Equal("Re: Backend Developer application update", payload.Subject);
         Assert.Contains("Maria", payload.Body);