refactor(analytics): extract AnalyticsService from JobApplicationsController

First Wave 2 (safe refactor) slice. Move the read-only stats/overview aggregation
out of the 3.3k-line JobApplicationsController into a dedicated, injectable
AnalyticsService, and lift its response DTOs (JobStats, FunnelStagePoint,
ResponseRatePoint, CompanyActivityPoint, AnalyticsOverviewDto) into
Models/AnalyticsDtos.cs.

- GetStats: ~44 lines -> 3 (delegates to AnalyticsService.GetStatsAsync).
- GetAnalyticsOverview: ~82 lines -> 3 (delegates to GetAnalyticsOverviewAsync).
- Registered AddScoped<AnalyticsService>(); controller keeps an optional ctor
  param with a `?? new AnalyticsService(db)` fallback so the 6 test sites that
  construct the controller directly keep compiling.
- Logic is byte-identical (same tenant-scoped context, same projections) so
  behaviour is preserved.

Backend suite: 92/92 green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

JobTrackerApi/Program.cs

@@ -153,6 +153,7 @@ builder.Services.AddHttpClient("ai-service", client =>
 });
 
 builder.Services.AddMemoryCache();
+builder.Services.AddScoped<AnalyticsService>();
 builder.Services.AddSingleton<ISummarizerService, SummarizerService>();
 builder.Services.AddSingleton<ICvAiClassifier, CvAiClassifier>();
 builder.Services.AddSingleton<ICvAiNormalizer, CvAiNormalizer>();