P2: Fix S3 multipart route - use query param for action

- Change route from /s3/multipart/:bucket/*key/init to /s3/multipart/:bucket/*key?action=init
- Add multipart_handler to unify all multipart operations
- Use Response type instead of impl IntoResponse for type compatibility

markbase-core/src/server.rs

@@ -245,11 +245,12 @@ pub async fn run(port: u16, file: Option<String>) -> anyhow::Result<()> {
             .post(crate::s3::put_object)  // POST for uploads (same handler handles multipart detection)
             .delete(crate::s3::delete_object)
         )
-        // Multipart upload endpoints
-        .route("/s3/multipart/:bucket/*key/init", post(crate::s3::initiate_multipart_upload))
-        .route("/s3/multipart/:bucket/*key/part", put(crate::s3::upload_part))
-        .route("/s3/multipart/:bucket/*key/complete", post(crate::s3::complete_multipart_upload))
-        .route("/s3/multipart/:bucket/*key/abort", delete(crate::s3::abort_multipart_upload))
+        // Multipart upload endpoints (use query param for action)
+        .route("/s3/multipart/:bucket/*key", 
+            post(crate::s3::multipart_handler)
+            .put(crate::s3::multipart_handler)
+            .delete(crate::s3::multipart_handler)
+        )
         // Bucket policy endpoints
         .route("/s3/policy/:bucket", get(crate::s3::get_bucket_policy))
         .route("/s3/policy/:bucket", put(crate::s3::put_bucket_policy))