Add single-image zk and hotlink upload API with service tests.

Replace batch DTOs with per-mode endpoints, persist ciphertext with file-type
checks, and return a hashed delete token for future revocation.

backend/src/modules/upload/dto/CreateHotlinkUploadDto.ts

@@ -1,19 +1,9 @@
-import { Type } from 'class-transformer';
-import { ArrayMaxSize, ArrayMinSize, IsArray, IsNotEmpty, ValidateNested } from 'class-validator';
-import { getAppConfig } from '../../../config';
-import { CreateHotlinkUploadItemDto } from './CreateHotlinkUploadItemDto';
-import { UploadBatchOptionsDto } from './UploadBatchOptionsDto';
+import { IsMimeType, IsNotEmpty, IsString } from 'class-validator';
+import { UploadCiphertextOptionsDto } from './UploadCiphertextOptionsDto';
 
-const {
-    validation: { maxImagesPerUpload }
-} = getAppConfig();
-
-export class CreateHotlinkUploadDto extends UploadBatchOptionsDto {
+export class CreateHotlinkUploadDto extends UploadCiphertextOptionsDto {
     @IsNotEmpty()
-    @IsArray()
-    @ArrayMinSize(1)
-    @ArrayMaxSize(maxImagesPerUpload)
-    @ValidateNested({ each: true })
-    @Type(() => CreateHotlinkUploadItemDto)
-    items: CreateHotlinkUploadItemDto[];
+    @IsString()
+    @IsMimeType()
+    mimeType: string;
 }