feat: add thumbnail metadata and download endpoint

- Extend `BoxFile` with thumbnail path/status fields and internal URL
- Populate `ThumbnailURL` when a thumbnail path is present during decoration
- Add `/box/:id/thumbnails/:file_id` route and handler to serve JPEG thumbnails
- Introduce thumbnail status constants to standardize processing state reportingfeat: add thumbnail metadata and download endpoint

- Extend `BoxFile` with thumbnail path/status fields and internal URL
- Populate `ThumbnailURL` when a thumbnail path is present during decoration
- Add `/box/:id/thumbnails/:file_id` route and handler to serve JPEG thumbnails
- Introduce thumbnail status constants to standardize processing state reporting

lib/models/models.go

@@ -9,6 +9,13 @@ const (
 	FileStatusWork   = "uploading"
 )
 
+const (
+	ThumbnailStatusFailed      = "failed"
+	ThumbnailStatusProcessing  = "processing"
+	ThumbnailStatusReady       = "ready"
+	ThumbnailStatusUnsupported = "unsupported"
+)
+
 type RetentionOption struct {
 	Key     string `json:"key"`
 	Label   string `json:"label"`
@@ -16,18 +23,21 @@ type RetentionOption struct {
 }
 
 type BoxFile struct {
-	ID           string `json:"id"`
-	Name         string `json:"name"`
-	Size         int64  `json:"size"`
-	SizeLabel    string `json:"size_label"`
-	MimeType     string `json:"mime_type"`
-	Status       string `json:"status"`
-	StatusLabel  string `json:"status_label"`
-	Title        string `json:"title"`
-	IconPath     string `json:"icon_path"`
-	DownloadPath string `json:"download_path"`
-	UploadPath   string `json:"upload_path"`
-	IsComplete   bool   `json:"is_complete"`
+	ID              string  `json:"id"`
+	Name            string  `json:"name"`
+	Size            int64   `json:"size"`
+	SizeLabel       string  `json:"size_label"`
+	MimeType        string  `json:"mime_type"`
+	Status          string  `json:"status"`
+	StatusLabel     string  `json:"status_label"`
+	Title           string  `json:"title"`
+	IconPath        string  `json:"icon_path"`
+	ThumbnailPath   *string `json:"thumbnail_path"`
+	ThumbnailStatus string  `json:"thumbnail_status,omitempty"`
+	ThumbnailURL    string  `json:"-"`
+	DownloadPath    string  `json:"download_path"`
+	UploadPath      string  `json:"upload_path"`
+	IsComplete      bool    `json:"is_complete"`
 }
 
 type BoxManifest struct {