Add dedicated blog image upload config separate from product thumbs.

Introduce MULTER_BLOG_IMAGE_* and VITE_BLOG_IMAGE_* env vars with backend multer helper and CMS config wiring for prod builds.

backend/src/config/validate.ts

@@ -75,6 +75,16 @@ class EnvironmentVariables {
     @Max(10 * 1024 * 1024) // 10MB
     MULTER_PRODUCT_THUMB_MAX_FILE_BYTES: number;
 
+    @IsNotEmpty()
+    @IsString()
+    MULTER_BLOG_IMAGE_ALLOWED_MIMES: string;
+
+    @IsNotEmpty()
+    @IsNumber()
+    @Min(1)
+    @Max(10 * 1024 * 1024) // 10MB
+    MULTER_BLOG_IMAGE_MAX_FILE_BYTES: number;
+
     @IsNotEmpty()
     @IsString()
     MULTER_SHOP_LOGO_ALLOWED_MIMES: string;