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/IEmailAccountRepository.cs

@@ -0,0 +1,24 @@
+using DigitalData.EmailProfiler.Domain.Entities;
+
+namespace DigitalData.EmailProfiler.Application.Interfaces.Repositories;
+
+/// <summary>
+/// Repository interface for EmailAccount entity.
+/// </summary>
+public interface IEmailAccountRepository : IRepository<EmailAccount>
+{
+    /// <summary>
+    /// Get all active email accounts.
+    /// </summary>
+    Task<IEnumerable<EmailAccount>> GetActiveAccountsAsync(CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Get email account by account name.
+    /// </summary>
+    Task<EmailAccount?> GetByAccountNameAsync(string accountName, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Get email account with its profiles.
+    /// </summary>
+    Task<EmailAccount?> GetWithProfilesAsync(int id, CancellationToken cancellationToken = default);
+}