feat: 异步图片任务结果落对象存储

为异步生图任务增加 S3 兼容对象存储支持，任务结果不再把大图内联存进 Redis：

- 新增可插拔接口 service.ImageStorage（Save -> url），适配别的厂商只需实现它
- S3 实现 S3ImageStorage（AWS S3 / R2 / 阿里云 OSS / MinIO），与备份共用 S3 客户端构造
- 新增 image_storage 配置（config.yaml + IMAGE_STORAGE_* 环境变量），默认关闭
- enabled 同时作为总开关：关闭或未配置对象存储时，异步生图接口返回 404 且不写
  Redis，从根上避免几 MB 的 b64_json 结果撑爆 Redis
- 完成时把图片上传对象存储并把结果改写为短链接（公开直链或 presigned），
  上传失败则任务标记为失败

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SM1tf3CFVRzC7guuhBXvMd

docs/ASYNC_IMAGE_TASKS.md

@@ -16,6 +16,31 @@ The aliases are `/images/generations/async`, `/images/edits/async`, and `/images
 
 Only OpenAI and Grok groups are supported. Requests use the same JSON or multipart payload as the corresponding synchronous endpoint. Streaming image requests are rejected because a polled task returns one final JSON result.
 
+## Enabling the feature (object storage)
+
+Asynchronous image tasks are **disabled by default** and gated on object storage. When the switch is off — or the S3 credentials are incomplete — the async endpoints return `404` and never create a task or write to Redis. This is deliberate: without offloading, large `b64_json` results (several MB each, e.g. `gpt-image-1`) would accumulate in Redis and exhaust its memory.
+
+Configure an S3-compatible object store (AWS S3, Cloudflare R2, Aliyun OSS, MinIO, …) in `config.yaml` (all keys also accept the `IMAGE_STORAGE_*` environment overrides):
+
+```yaml
+image_storage:
+  enabled: true
+  endpoint: "https://<account_id>.r2.cloudflarestorage.com"  # AWS 官方可留空
+  region: "auto"
+  bucket: "my-images"
+  access_key_id: "..."
+  secret_access_key: "..."
+  prefix: "images/"
+  force_path_style: false          # MinIO/path-style buckets set true
+  public_base_url: ""              # set to return public_base_url/key直链; empty → presigned URL
+  presign_expiry_hours: 24         # presigned link TTL when public_base_url is empty
+  max_download_bytes: 33554432     # cap when re-hosting an upstream image URL (32MB)
+```
+
+When a task completes, each generated image is uploaded to the bucket and the result is rewritten to a compact form: `data[].url` points at the stored object (a permanent `public_base_url/key` link, or a time-limited presigned URL) and `b64_json` is removed. Only this small JSON is stored in Redis. If an upload fails, the task is marked `failed` rather than persisting the raw base64.
+
+To support a different vendor beyond the S3-compatible client, implement the `service.ImageStorage` interface (`Save(ctx, key, contentType, data) (url, error)`) and provide it in place of the S3 implementation.
+
 ## Submit a task
 
 ```bash
@@ -67,7 +92,7 @@ While work is in progress:
 }
 ```
 
-On success, `result` is the unmodified JSON body from the synchronous image API, so URL and base64 response formats both remain supported:
+On success, `result` mirrors the synchronous image API body, except each image has been offloaded to object storage: `data[].url` points at the stored object and `b64_json` is stripped (so both URL and base64 upstream formats end up as compact stored links):
 
 ```json
 {