feat(application): add repository and service interfaces

Repository Interfaces (Clean Architecture - Application Layer):
- IRepository<T>: Base repository interface with common CRUD operations
- IEmailAccountRepository: Email account operations (GetActive, GetByName, GetWithProfiles)
- IEmailProfileRepository: Profile operations (GetActive, GetDueForPolling, GetWithRelated)
- IEmailProcessRepository: Process operations (GetWithSteps, GetByType)
- IEmailHistoryRepository: History operations (pagination, duplicate detection, date range queries)
- IEmailOutboxRepository: Outbox operations (GetPending, GetForRetry, MarkAsSent/Failed)
- IUnitOfWork: Transaction management and repository aggregation

Service Interfaces (Abstraction for Infrastructure):
- IEmailService: IMAP/SMTP operations with OAuth2 support (MailKit wrapper)
- IPdfProcessingService: PDF validation, embedded file extraction, ZUGFeRD support
- IDmsService: windream DMS integration (archive, search, update index fields)
- IEncryptionService: Data protection for passwords and OAuth tokens
- IEmailQueue: Async email queue (in-memory Channel, future: RabbitMQ)

Dependencies:
- Added MimeKit 4.17.0 for email service interface definitions

All interfaces follow Clean Architecture principles:
- Interfaces in Application layer
- Implementations will be in Infrastructure layer

src/DigitalData.EmailProfiler.Application/Interfaces/Repositories/IRepository.cs

@@ -0,0 +1,19 @@
+using System.Linq.Expressions;
+
+namespace DigitalData.EmailProfiler.Application.Interfaces.Repositories;
+
+/// <summary>
+/// Base repository interface for common CRUD operations.
+/// </summary>
+/// <typeparam name="T">Entity type</typeparam>
+public interface IRepository<T> where T : class
+{
+    Task<T?> GetByIdAsync(int id, CancellationToken cancellationToken = default);
+    Task<IEnumerable<T>> GetAllAsync(CancellationToken cancellationToken = default);
+    Task<IEnumerable<T>> FindAsync(Expression<Func<T, bool>> predicate, CancellationToken cancellationToken = default);
+    Task<T> AddAsync(T entity, CancellationToken cancellationToken = default);
+    Task UpdateAsync(T entity, CancellationToken cancellationToken = default);
+    Task DeleteAsync(T entity, CancellationToken cancellationToken = default);
+    Task<bool> ExistsAsync(Expression<Func<T, bool>> predicate, CancellationToken cancellationToken = default);
+    Task<int> CountAsync(Expression<Func<T, bool>>? predicate = null, CancellationToken cancellationToken = default);
+}