Add blog thumbnail and banner upload endpoints with storage keys.

Store image filenames in DB, resolve paths for cleanup on replace/delete, and reuse product-thumb multer limits.

backend/src/database/migrations/1784900000000-add-blog.ts

@@ -5,7 +5,7 @@ export class AddBlog1784900000000 implements MigrationInterface {
 
     public async up(queryRunner: QueryRunner): Promise<void> {
         await queryRunner.query(
-            `CREATE TABLE "blog_posts" ("id" uuid NOT NULL DEFAULT uuid_generate_v4(), "title" character varying NOT NULL, "thumbnailUrl" character varying, "bannerUrl" character varying, "contentHtml" text NOT NULL, "isDraft" boolean NOT NULL DEFAULT true, "createdAt" TIMESTAMP NOT NULL DEFAULT now(), "updatedAt" TIMESTAMP NOT NULL DEFAULT now(), CONSTRAINT "PK_blog_posts_id" PRIMARY KEY ("id"))`
+            `CREATE TABLE "blog_posts" ("id" uuid NOT NULL DEFAULT uuid_generate_v4(), "title" character varying NOT NULL, "thumbnailStorageKey" character varying, "bannerStorageKey" character varying, "contentHtml" text NOT NULL, "isDraft" boolean NOT NULL DEFAULT true, "createdAt" TIMESTAMP NOT NULL DEFAULT now(), "updatedAt" TIMESTAMP NOT NULL DEFAULT now(), CONSTRAINT "PK_blog_posts_id" PRIMARY KEY ("id"))`
         );
         await queryRunner.query(`ALTER TABLE "shop_settings" ADD "blogEnabled" boolean NOT NULL DEFAULT false`);
     }