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/dto/requests/document.go

@@ -25,12 +25,11 @@ type CreateDocumentRequest struct {
 	SerialNumber  string               `json:"serial_number" validate:"max=100"`
 	ModelNumber   string               `json:"model_number" validate:"max=100"`
 	TaskID        *uint                `json:"task_id"`
-	ImageURLs     []string             `json:"image_urls" validate:"omitempty,max=20,dive,max=500"` // Legacy multipart upload path
 	// UploadIDs claims pending_uploads rows produced by the presigned-URL
-	// upload flow and turns them into document_image rows. May be combined
-	// with ImageURLs during the rollout window. UploadIDs of category
-	// "document_file" attach to the document's main FileURL/FileName fields
-	// instead — the service infers placement from the row's category.
+	// upload flow and turns them into document_image rows. UploadIDs of
+	// category "document_file" attach to the document's main FileURL +
+	// FileName fields instead — the service infers placement from the
+	// row's category.
 	UploadIDs     []uint               `json:"upload_ids" validate:"omitempty,max=20"`
 }