refactor(uploads): drop legacy multipart code paths

The presigned-URL upload flow (POST /api/uploads/presign + direct B2 POST
+ upload_ids[] in entity creation) is now the only image upload path. The
legacy multipart routes and DTO fields used by older clients are removed:

Removed:
  - POST /api/uploads/image/        (legacy multipart upload → URL)
  - POST /api/uploads/document/     (legacy multipart upload → URL)
  - POST /api/uploads/completion/   (legacy multipart upload → URL)
  - Multipart branch in POST /api/task-completions/ (now JSON-only)
  - CreateTaskCompletionRequest.ImageURLs DTO field
  - UpdateTaskCompletionRequest.ImageURLs DTO field
  - CreateDocumentRequest.ImageURLs DTO field
  - Service-layer ImageURLs loops in task_service.CreateCompletion,
    task_service.UpdateCompletion, document_service.CreateDocument
  - Tests exercising the removed paths
  - Now-unused imports (strings/time/decimal) in task_handler.go

Kept:
  - DELETE /api/uploads/  (orphan-cleanup endpoint, still useful)
  - POST /api/uploads/presign/  (the new path)
  - POST /api/documents/:id/images/  (uses storage_service.Upload directly,
    same multipart pattern but separate code path; deferred for now)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/services/document_service.go

@@ -204,21 +204,7 @@ func (s *DocumentService) CreateDocument(ctx context.Context, req *requests.Crea
 		return nil, apperrors.Internal(err)
 	}
 
-	// Legacy multipart path — already-uploaded URLs.
-	for _, imageURL := range req.ImageURLs {
-		if imageURL != "" {
-			img := &models.DocumentImage{
-				DocumentID: document.ID,
-				ImageURL:   imageURL,
-			}
-			if err := s.documentRepo.WithContext(ctx).CreateDocumentImage(img); err != nil {
-				// Log but don't fail the whole operation
-				continue
-			}
-		}
-	}
-
-	// New presigned path — claimed image uploads become DocumentImage rows.
+	// Presigned-URL path — claimed image uploads become DocumentImage rows.
 	// The document_file row (if any) was already lifted onto the document above.
 	for i := range claimedUploads {
 		pu := claimedUploads[i]