docs: align with new storage architecture (s3 + local + server-proxied PUT)

All operator- and developer-facing docs updated to reflect:

  - The unified AsyncStorageBackend abstraction (s3 + local backends).
  - The STORAGE_BACKEND toggle ("s3" default, "local" for dev /
    single-node / air-gapped deployments).
  - The 4-purpose-bucket layout (workspace / version / run_log / trash)
    in both modes — 4 separate S3 buckets in s3 mode, 4 subdirectories
    of LOCAL_STORAGE_BASE_DIR in local mode.
  - The S3_* env var naming (was RUSTFS_*).
  - The server-proxied upload flow (was browser-direct presign-PUT):
    POST /internal/v1/uploads → PUT /internal/v1/uploads/{id} with
    raw bytes → server calls backend.put().
  - The factory helpers workspaces_root() (runtime's view of the
    workspace bucket on disk) and rclone_remote_spec() (s3-mode mount
    source).
  - The "two settings describing the same thing" cleanup: the deleted
    settings.workspace_root, settings.workspaces_root, and
    settings.remote_bucket fields.

Files touched:
  - API.md (§5 data-resource upload flow, §9 storage control plane,
    §10 readiness example)
  - ARCHITECTURE.md (storage layer diagram)
  - CLAUDE.md (architecture description + volume-preservation note)
  - DEVELOP.md (settings list, Storage section, "Wire a new bucket"
    how-to, dev-export example, troubleshooting network hint)
  - README.md (architecture diagram, container table, quick-start
    credentials note, tear-down note, Storage layout section)
  - REFACTOR_NOTES.md (final container list with s3 explanation)
  - backend/README.md (storage backend description)
  - migrations/data/README.md (step 11/12 record mentioning object
    storage)

A handful of historical "RustFS" mentions are intentionally retained
where they name a specific S3-compatible product (e.g. as an example
in REFACTOR_NOTES.md's container list) or document the pre-2026
abstraction name (DEVELOP.md Storage section).

backend/README.md

@@ -1,5 +1,8 @@
 # Backend
 
 统一 FastAPI 管理服务。包含用户、Workspace、脚本、稳定版本、调度定义、
-立即运行以及 RustFS 对象接口。原 `platform_api` 与 `storage_api` 已在此
+立即运行以及 S3 对象接口。原 `platform_api` 与 `storage_api` 已在此
 模块合并，外部 REST 契约保持不变。
+
+底层走的是 `common.storage.AsyncStorageBackend` 抽象，按
+`settings.storage_backend` 切换 s3 / local 两种实现。