fix(storage): correct presign content-type claim; head returns contentType

Spec review (live negative test) found s3-request-presigner unconditionally
unsigns content-type: header-less PUT returns 200, so the previous docblock
claiming signature enforcement was a false guarantee. Content-Type is now
documented as convention-only; head() returns contentType so the complete
step (Task 19) can verify it server-side against the session declaration.

Co-Authored-By: Claude Code <noreply@anthropic.com>

packages/storage/src/minio.adapter.ts

@@ -46,10 +46,12 @@ export class MinioStorageAdapter implements StoragePort {
   }
 
   /**
-   * 预签名上传。S3 预签名无法对 Content-Length 签名（SigV4 规范排除该头），
-   * bytes 参数仅作为端口契约保留：字节数核验发生在上传完成后的 head/verify 阶段。
-   * Content-Type 经 PutObjectCommand 的 ContentType 参数参与签名，客户端必须
-   * 原样携带返回的 headers，否则 403 SignatureDoesNotMatch。
+   * 预签名上传。SigV4 预签名无法对 Content-Length 签名（规范排除）；所装版本的
+   * s3-request-presigner 亦将 Content-Type 列为不可签名头（prepareRequest 强制
+   * unsignableHeaders.add("content-type")，实测不带头的 PUT 返回 200）。
+   * 因此返回的 headers 是「约定必须发送」而非签名强制：字节完整性由 verify 阶段
+   * 的 head 核验，Content-Type 一致性由 complete 时服务端比对 head().contentType
+   * 与会话申明值兜底（见 Task 19）。bytes 参数仅作为端口契约保留。
    */
   async presignPut(objectKey: string, bytes: number, contentType: string, ttlSeconds: number): Promise<UploadCredential> {
     void bytes; // 见上：签名不含 Content-Length，大小核验由 verify 阶段负责
@@ -73,10 +75,10 @@ export class MinioStorageAdapter implements StoragePort {
     });
   }
 
-  async head(objectKey: string): Promise<{ bytes: number; etag: string } | null> {
+  async head(objectKey: string): Promise<{ bytes: number; etag: string; contentType: string } | null> {
     try {
       const out = await this.client.send(new HeadObjectCommand({ Bucket: this.bucket, Key: objectKey }));
-      return { bytes: out.ContentLength ?? 0, etag: out.ETag ?? '' };
+      return { bytes: out.ContentLength ?? 0, etag: out.ETag ?? '', contentType: out.ContentType ?? '' };
     } catch (error) {
       if (isNotFound(error)) return null;
       throw error;