Phase 13: Migrate AdminDashboard controller to FastEndpoints

- Create AdminDashboardDtos.cs with request/response types
- Migrate 4 endpoints: GetSummaryEndpoint, GetUpcomingFilingsEndpoint,
  GetRecentInquiriesEndpoint, GetMonthlyStatsEndpoint
- Remove legacy AdminDashboardController.cs
- Maintain API path compatibility (/api/admin-dashboard/*)
- All endpoints use FastEndpoints uniform pattern

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/TaxBaik.Web/Endpoints/AdminDashboard/GetMonthlyStatsEndpoint.cs

@@ -36,7 +36,7 @@ public class GetMonthlyStatsEndpoint : Endpoint<MonthlyStatsQuery, MonthlyStatsR
                 CompletionRate = statsDict.completionRate
             }, 200, cancellation: ct);
         }
-        catch (Exception ex)
+        catch (Exception)
         {
             await SendErrorsAsync(500, ct);
         }

src/TaxBaik.Web/Endpoints/AdminDashboard/GetRecentInquiriesEndpoint.cs

@@ -32,7 +32,7 @@ public class GetRecentInquiriesEndpoint : Endpoint<RecentInquiriesQuery, RecentI
                 Limit = limit
             }, 200, cancellation: ct);
         }
-        catch (Exception ex)
+        catch (Exception)
         {
             await SendErrorsAsync(500, ct);
         }

src/TaxBaik.Web/Endpoints/AdminDashboard/GetSummaryEndpoint.cs

@@ -32,7 +32,7 @@ public class GetSummaryEndpoint : EndpointWithoutRequest<AdminDashboardSummaryRe
                 RecentInquiries = summary.RecentInquiries.ToList()
             }, 200, cancellation: ct);
         }
-        catch (Exception ex)
+        catch (Exception)
         {
             await SendErrorsAsync(500, ct);
         }

src/TaxBaik.Web/Endpoints/AdminDashboard/GetUpcomingFilingsEndpoint.cs

@@ -30,7 +30,7 @@ public class GetUpcomingFilingsEndpoint : Endpoint<UpcomingFilingsQuery, Upcomin
                 Days = days
             }, 200, cancellation: ct);
         }
-        catch (Exception ex)
+        catch (Exception)
         {
             await SendErrorsAsync(500, ct);
         }