feat: implement debounced autosave with strict LocalStorage garbage collection (Stage 2 Task B)

src/NexusReader.Application/Common/AppJsonContext.cs

@@ -21,6 +21,8 @@ namespace NexusReader.Application.Common;
 [JsonSerializable(typeof(NexusReader.Application.DTOs.Media.ValidateChapterRequest))]
 [JsonSerializable(typeof(NexusReader.Application.DTOs.Media.ValidateChapterResponse))]
 [JsonSerializable(typeof(NexusReader.Application.DTOs.Media.UploadResultDto))]
+[JsonSerializable(typeof(NexusReader.Application.DTOs.Media.LocalBackupEnvelope))]
+[JsonSerializable(typeof(NexusReader.Application.DTOs.Media.AutosaveChapterRequest))]
 public partial class AppJsonContext : JsonSerializerContext
 {
 }

src/NexusReader.Application/DTOs/Media/MediaDtos.cs

@@ -16,3 +16,18 @@ public record ValidateChapterResponse(string SanitizedContent);
 /// Response DTO containing the uploaded media file URL.
 /// </summary>
 public record UploadResultDto(string Url);
+
+/// <summary>
+/// Represents a structured JSON backup envelope stored in LocalStorage.
+/// </summary>
+public class LocalBackupEnvelope
+{
+    public Guid ChapterId { get; set; }
+    public DateTime Timestamp { get; set; }
+    public string MarkdownContent { get; set; } = string.Empty;
+}
+
+/// <summary>
+/// Request DTO for chapter autosaving.
+/// </summary>
+public record AutosaveChapterRequest(string MarkdownContent);