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>

Models/AnalyticsDtos.cs

@@ -0,0 +1,30 @@
+using System.Collections.Generic;
+
+namespace JobTrackerApi.Models
+{
+    // Read-only analytics/statistics response DTOs. Extracted from
+    // JobApplicationsController so the aggregation logic can live in AnalyticsService.
+    public sealed record JobStats(
+        int Total,
+        int Active,
+        int Deleted,
+        Dictionary<string, int> ByStatus,
+        int AppliedLast30Days,
+        double AverageDaysSinceApplied
+    );
+
+    public sealed record FunnelStagePoint(string Label, int Count);
+
+    public sealed record ResponseRatePoint(string Label, int Total, int Responses, double Rate);
+
+    public sealed record CompanyActivityPoint(int CompanyId, string Company, int Count, int Responses, double ResponseRate);
+
+    public sealed record AnalyticsOverviewDto(
+        List<FunnelStagePoint> Funnel,
+        List<ResponseRatePoint> ResponseRateBySource,
+        List<CompanyActivityPoint> TopCompanies,
+        double? MedianDaysToFirstResponse,
+        int TotalResponses,
+        int TotalActive
+    );
+}