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/router/router.go

@@ -735,9 +735,6 @@ func setupUserRoutes(api *echo.Group, userHandler *handlers.UserHandler) {
 func setupUploadRoutes(api *echo.Group, uploadHandler *handlers.UploadHandler) {
 	uploads := api.Group("/uploads")
 	{
-		uploads.POST("/image/", uploadHandler.UploadImage)
-		uploads.POST("/document/", uploadHandler.UploadDocument)
-		uploads.POST("/completion/", uploadHandler.UploadCompletion)
 		uploads.POST("/presign/", uploadHandler.PresignUpload)
 		uploads.DELETE("/", uploadHandler.DeleteFile)
 	}