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/Services/IPdfProcessingService.cs

@@ -0,0 +1,59 @@
+namespace DigitalData.EmailProfiler.Application.Interfaces.Services;
+
+/// <summary>
+/// PDF processing service interface for validation and embedded file extraction.
+/// </summary>
+public interface IPdfProcessingService
+{
+    /// <summary>
+    /// Validate if file is a valid PDF.
+    /// </summary>
+    Task<bool> IsValidPdfAsync(string filePath, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Validate if stream contains a valid PDF.
+    /// </summary>
+    Task<bool> IsValidPdfAsync(Stream stream, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Extract embedded files from PDF (e.g., ZUGFeRD XML).
+    /// </summary>
+    Task<IEnumerable<EmbeddedFile>> ExtractEmbeddedFilesAsync(string pdfPath, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Extract embedded files from PDF stream.
+    /// </summary>
+    Task<IEnumerable<EmbeddedFile>> ExtractEmbeddedFilesAsync(Stream pdfStream, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Check if PDF contains ZUGFeRD data.
+    /// </summary>
+    Task<bool> HasZugFeRDDataAsync(string pdfPath, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Extract ZUGFeRD XML from PDF.
+    /// </summary>
+    Task<string?> ExtractZugFeRDXmlAsync(string pdfPath, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Get PDF metadata (title, author, creation date, etc.).
+    /// </summary>
+    Task<PdfMetadata> GetMetadataAsync(string pdfPath, CancellationToken cancellationToken = default);
+}
+
+/// <summary>
+/// Represents an embedded file extracted from a PDF.
+/// </summary>
+public record EmbeddedFile(string FileName, byte[] Content, string? Description = null);
+
+/// <summary>
+/// Represents PDF metadata.
+/// </summary>
+public record PdfMetadata(
+    string? Title,
+    string? Author,
+    string? Subject,
+    string? Keywords,
+    DateTime? CreationDate,
+    DateTime? ModificationDate,
+    int PageCount);