fix: resolve SonarQube hotspots for upload size and X-Powered-By

- S5693: MAX_IMAGE_BYTES was 8 * 1024 * 1024 (8,388,608), just over the
  8,000,000-byte ceiling the rule treats as safe, so the hotspot on the
  multer storage config never cleared. Use 8_000_000.
- S5689: Express advertises its stack in X-Powered-By by default, which
  tells an attacker what to aim exploits at. Disable the header.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/src/routes/admin.ts

@@ -13,7 +13,9 @@ const UPLOADS_DIR = process.env.UPLOADS_DIR || '/app/uploads';
 // multipart body: image count, bytes per image, and the small text fields
 // (name/description/price) that accompany them.
 const MAX_IMAGES_PER_REQUEST = 6;
-const MAX_IMAGE_BYTES = 8 * 1024 * 1024;
+// 8 MB, not 8 MiB — this is the ceiling S5693 treats as safe, and 8 * 1024 *
+// 1024 sits just over it. Plenty for a product photo either way.
+const MAX_IMAGE_BYTES = 8_000_000;
 const MAX_TEXT_FIELDS = 8;
 const MAX_TEXT_FIELD_BYTES = 64 * 1024;