refactor(upload): use IncomingFile interface instead of multipart headers

Refactors the upload handler to use the `services.IncomingFile` interface instead of concrete `*multipart.FileHeader` pointers. This decouples the core upload logic from the HTTP multipart implementation, allowing for more flexible file sources.

Changes include:
- Introducing `namedMultipartFile` to adapt multipart headers to the new interface.
- Updating `createOrAppendBox`, `checkUploadPolicy`, and `totalUploadBytes` to accept `IncomingFile`.
- Renaming service calls to `CreateBoxFromIncoming` and `AppendIncomingFiles`.

backend/libs/jobs/thumbnails.go

@@ -22,6 +22,7 @@ import (
 	"sort"
 	"strconv"
 	"strings"
+	"sync"
 	"time"
 
 	"golang.org/x/image/font"
@@ -38,8 +39,12 @@ type ThumbnailJobResult struct {
 	Failed    int
 }
 
+var thumbnailJobs sync.WaitGroup
+
 func GenerateThumbnailsForBoxAsync(uploadService *services.UploadService, logger *slog.Logger, boxID string) {
+	thumbnailJobs.Add(1)
 	go func() {
+		defer thumbnailJobs.Done()
 		box, err := uploadService.GetBox(boxID)
 		if err != nil {
 			logger.Warn("thumbnail box lookup failed", "source", "thumbnail", "severity", "warn", "code", 4204, "box_id", boxID, "error", err.Error())
@@ -61,6 +66,10 @@ func GenerateThumbnailsForBoxAsync(uploadService *services.UploadService, logger
 	}()
 }
 
+func WaitForThumbnailJobs() {
+	thumbnailJobs.Wait()
+}
+
 func newThumbnailsJob(cfg config.Config, logger *slog.Logger, uploadService *services.UploadService) job {
 	return job{
 		name:     "thumbnail",