refactor(arch): introduce IEbookRepository, ISyncBroadcaster, fix EpubReader path resolution

Critical fixes (review findings #1, #2, #3):
- Create IEbookRepository abstraction in Application layer
- Remove illegal EF Core dependency from IngestEbookCommandHandler
- Create EbookRepository implementation in Infrastructure/Persistence
- Create ISyncBroadcaster in Application/Abstractions/Messaging
- Create SignalRSyncBroadcaster in Infrastructure/RealTime
- Move UpdateReadingProgressCommandHandler from Infrastructure → Application
- Add EbookId to GetReaderPageQuery and IEpubReader signature
- Rewrite EpubReaderService: DB-resolved file path, remove auto-provisioning
- Split EpubService.cs into EpubReaderService.cs + EpubMetadataExtractor.cs
- Add CurrentEbookId to IReaderNavigationService and ReaderNavigationService
- Update WasmEpubReader and /api/epub endpoint for new signature

High severity fixes (#4, #6, #7, #8, #16):
- Change BookStorageService registration from Singleton → Scoped
- Fix empty catch{} in ReaderCanvas JS interop init — now logs warnings
- Replace all Console.WriteLine with ILogger in KnowledgeService + ReaderCanvas
- Cache JsonSerializerOptions as static field in KnowledgeService
- Wrap SyncService Task.Run body in comprehensive try/catch with ILogger

Medium/Low fixes (#11, #13, #14, #15, #18, #20):
- BookIngestionModal.DisposeAsync now nullifies _epubBytes (50MB array)
- KnowledgeCoordinator.OnGraphUpdated: Action<T> → Func<T, Task>
- BookStorageService: Path.Combine → forward-slash string interpolation
- SignalR CancellationToken passed as named parameter (not payload arg)

src/NexusReader.Application/Commands/Library/IngestEbookCommandHandler.cs

@@ -1,30 +1,27 @@
 using FluentResults;
 using MediatR;
-using Microsoft.EntityFrameworkCore;
 using NexusReader.Application.Abstractions.Messaging;
+using NexusReader.Application.Abstractions.Persistence;
 using NexusReader.Application.Abstractions.Services;
-using NexusReader.Data.Persistence;
 using NexusReader.Domain.Entities;
 
 namespace NexusReader.Application.Commands.Library;
 
 public class IngestEbookCommandHandler : IRequestHandler<IngestEbookCommand, Result<Guid>>
 {
-    private readonly IDbContextFactory<AppDbContext> _dbContextFactory;
+    private readonly IEbookRepository _ebookRepository;
     private readonly IBookStorageService _storageService;
 
     public IngestEbookCommandHandler(
-        IDbContextFactory<AppDbContext> dbContextFactory,
+        IEbookRepository ebookRepository,
         IBookStorageService storageService)
     {
-        _dbContextFactory = dbContextFactory;
+        _ebookRepository = ebookRepository;
         _storageService = storageService;
     }
 
     public async Task<Result<Guid>> Handle(IngestEbookCommand request, CancellationToken cancellationToken)
     {
-        using var context = await _dbContextFactory.CreateDbContextAsync(cancellationToken);
-        
         string epubPath;
         string? coverUrl;
 
@@ -36,6 +33,10 @@ public class IngestEbookCommandHandler : IRequestHandler<IngestEbookCommand, Res
                 ? await _storageService.SaveCoverAsync(request.CoverImage, $"{request.Title}_cover.jpg")
                 : null;
         }
+        catch (IOException ex)
+        {
+            return Result.Fail(new Error($"Storage I/O failure: {ex.Message}").CausedBy(ex));
+        }
         catch (Exception ex)
         {
             return Result.Fail(new Error($"Storage failure: {ex.Message}").CausedBy(ex));
@@ -43,17 +44,16 @@ public class IngestEbookCommandHandler : IRequestHandler<IngestEbookCommand, Res
 
         try
         {
-            // 2. Resolve Author
-            var authorName = string.IsNullOrWhiteSpace(request.AuthorName) ? "Unknown Author" : request.AuthorName.Trim();
-            
-            // Use case-insensitive comparison
-            var author = await context.Authors
-                .FirstOrDefaultAsync(a => a.Name.ToLower() == authorName.ToLower(), cancellationToken);
+            // 2. Resolve Author (case-insensitive via repository)
+            var authorName = string.IsNullOrWhiteSpace(request.AuthorName)
+                ? "Unknown Author"
+                : request.AuthorName.Trim();
 
+            var author = await _ebookRepository.FindAuthorByNameAsync(authorName, cancellationToken);
             if (author == null)
             {
                 author = new Author { Name = authorName };
-                context.Authors.Add(author);
+                _ebookRepository.AddAuthor(author);
             }
 
             // 3. Create Ebook
@@ -61,25 +61,21 @@ public class IngestEbookCommandHandler : IRequestHandler<IngestEbookCommand, Res
             {
                 Title = request.Title,
                 Author = author,
-                FilePath = epubPath, // Relative URL from wwwroot
+                FilePath = epubPath,
                 CoverUrl = coverUrl,
                 UserId = request.UserId,
                 TenantId = request.TenantId,
                 AddedDate = DateTime.UtcNow
             };
 
-            context.Ebooks.Add(ebook);
-            await context.SaveChangesAsync(cancellationToken);
+            _ebookRepository.AddEbook(ebook);
+            await _ebookRepository.SaveChangesAsync(cancellationToken);
 
             return Result.Ok(ebook.Id);
         }
-        catch (DbUpdateException ex)
+        catch (Exception ex)
         {
             return Result.Fail(new Error($"Database error during ingestion: {ex.Message}").CausedBy(ex));
         }
-        catch (Exception ex)
-        {
-            return Result.Fail(new Error($"Unexpected error during ingestion: {ex.Message}").CausedBy(ex));
-        }
     }
 }