feat: implement API-first architecture Phase 1 - Dashboard API

**Architecture Refactor (SOLID Principles):**
- Implement AdminDashboardController (REST API)
- Add dashboard summary endpoint
- Add upcoming filings endpoint
- Add recent inquiries endpoint
- Add monthly statistics endpoint

**Database Layer (Repository Pattern):**
- Extend IInquiryRepository with date range queries
- Implement CountByDateRangeAsync
- Implement CountByStatusAndDateAsync
- Extend InquiryRepository with new methods

**Service Layer (Single Responsibility):**
- Extend AdminDashboardService with API methods
- Add GetRecentInquiriesAsync
- Add GetMonthlyStatsAsync with caching

**Test Coverage:**
- Update FakeInquiryRepository mock with new methods

**SOLID Application:**
✓ Single Responsibility: Each class has one reason to change
✓ Open/Closed: Dashboard API can be extended without modifying existing code
✓ Dependency Inversion: Service depends on Repository abstraction
✓ Interface Segregation: API endpoints are focused and specific

Status: ✓ Compiles successfully (0 errors, 0 warnings)

Next phases:
- Add remaining API controllers (Announcement, Client, FAQ, TaxFiling)
- Refactor Blazor components to use API instead of services
- Implement JWT token refresh mechanism
- Add SignalR for change notifications

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

TaxBaik.Application.Tests/InquiryServiceTests.cs

@@ -58,6 +58,12 @@ public class InquiryServiceTests
         public Task<int> CountByStatusAsync(string status, CancellationToken cancellationToken = default)
             => Task.FromResult(Inquiries.Count(x => x.Status == status));
 
+        public Task<int> CountByDateRangeAsync(DateTime startDate, DateTime endDate, CancellationToken cancellationToken = default)
+            => Task.FromResult(Inquiries.Count(x => x.CreatedAt >= startDate && x.CreatedAt <= endDate));
+
+        public Task<int> CountByStatusAndDateAsync(string status, DateTime startDate, DateTime endDate, CancellationToken cancellationToken = default)
+            => Task.FromResult(Inquiries.Count(x => x.Status == status && x.CreatedAt >= startDate && x.CreatedAt <= endDate));
+
         public Task UpdateStatusAsync(int id, string status, CancellationToken cancellationToken = default)
         {
             var inquiry = Inquiries.FirstOrDefault(x => x.Id == id);