chore(analytics): remove dead day-hour heatmap endpoint/service/DTO

Unused after the frontend widget removal (#44); category-heatmap kept. Build+tests green.

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

src/InboxIntel.Api/Controllers/AnalyticsController.cs

@@ -24,9 +24,6 @@ public class AnalyticsController : ApiControllerBase
     public async Task<IActionResult> Volume([FromQuery] int days = 90, CancellationToken ct = default)
         => Ok(await _analytics.GetVolumeOverTimeAsync(UserId, Math.Clamp(days, 1, 3660), ct));
 
-    [HttpGet("heatmap")]
-    public async Task<IActionResult> Heatmap(CancellationToken ct) => Ok(await _analytics.GetHeatmapAsync(UserId, ct));
-
     [HttpGet("category-heatmap")]
     public async Task<IActionResult> CategoryHeatmap(CancellationToken ct) => Ok(await _analytics.GetCategoryHeatmapAsync(UserId, ct));