Expand skin upload schema and simplify texture storage

Updated the skin upload schema to accept both JSON and multipart/form-data content types, and to allow requests without a URL. Simplified the texture storage path in userService.js by removing subdirectory partitioning based on hash.

services/userService.js

@@ -492,8 +492,7 @@ async function uploadSkin(uuid, fileObject, variant) {
         const existingTexture = await userRepository.getTextureByHash(hash)
 
         if (!existingTexture) {
-            const subDir = hash.substring(0, 2)
-            const targetDir = path.join(TEXTURES_DIR, subDir)
+            const targetDir = path.join(TEXTURES_DIR)
             const targetPath = path.join(targetDir, hash)
 
             await fs.mkdir(targetDir, { recursive: true })