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/index.ts

@@ -128,6 +128,17 @@ export const getProductThumbMulterConfig = (): MulterConfig => {
     return { allowedMimes, maxFileBytes };
 };
 
+export const getBlogImageMulterConfig = (): MulterConfig => {
+    const allowedMimes = env('MULTER_BLOG_IMAGE_ALLOWED_MIMES')
+        .split(',')
+        .map(v => v.trim())
+        .filter(Boolean);
+
+    const maxFileBytes = envInt('MULTER_BLOG_IMAGE_MAX_FILE_BYTES');
+
+    return { allowedMimes, maxFileBytes };
+};
+
 export const getShopLogoMulterConfig = (): MulterConfig => {
     const allowedMimes = env('MULTER_SHOP_LOGO_ALLOWED_MIMES')
         .split(',')