feat: foundation for CRM and tax accounting specialized features

Domain Layer (SOLID Foundation):
- 5 New Entities: TaxProfile, TaxFilingSchedule, ConsultingActivity, Contract, RevenueTracking
- Client entity extended with tax-specific fields
- Multi-tenant support (company_id)

Database Migration (V015):
- Create tax_profiles table for detailed tax info
- Create tax_filing_schedules for deadline tracking
- Create consulting_activities for CRM (activity history)
- Create contracts for contract management
- Create revenue_tracking for invoice and payment tracking
- Add indexes for performance optimization

Repository Interfaces:
- ITaxProfileRepository (tax profile CRUD + risk analysis)
- ITaxFilingScheduleRepository (schedule management + deadline tracking)
- IConsultingActivityRepository (CRM activity tracking)
- IContractRepository (contract lifecycle + MRR calculation)
- IRevenueTrackingRepository (invoice + payment tracking + revenue analysis)

Architecture:
- Follows Repository Pattern with clear separation of concerns
- SOLID principles: each repo = one responsibility
- Extensible design for multi-tenant support
- Supports specialized tax accounting and CRM workflows

TaxBaik.Domain/Interfaces/IRevenueTrackingRepository.cs

@@ -0,0 +1,14 @@
+namespace TaxBaik.Domain.Interfaces;
+
+using TaxBaik.Domain.Entities;
+
+public interface IRevenueTrackingRepository
+{
+    Task<int> CreateAsync(RevenueTracking revenue, CancellationToken cancellationToken = default);
+    Task<IEnumerable<RevenueTracking>> GetByClientIdAsync(int clientId, CancellationToken cancellationToken = default);
+    Task<IEnumerable<RevenueTracking>> GetPendingPaymentsAsync(CancellationToken cancellationToken = default);
+    Task<IEnumerable<RevenueTracking>> GetByDateRangeAsync(DateTime startDate, DateTime endDate, CancellationToken cancellationToken = default);
+    Task UpdateAsync(RevenueTracking revenue, CancellationToken cancellationToken = default);
+    Task MarkPaidAsync(int id, DateTime paymentDate, CancellationToken cancellationToken = default);
+    Task<decimal> GetTotalRevenueAsync(DateTime startDate, DateTime endDate, CancellationToken cancellationToken = default);
+}