storage: extract unified AsyncStorageBackend abstraction + migrate from RustFS

Replace the old RustFS-specific storage layer (common.storage.client /
RustFSObjectStore) with a minimal sync/async abstraction:

  AsyncStorageBackend: put / get / get_stream / delete / exists / stat /
                       list / get_url / copy
  StorageBackend:      same surface, sync implementations
  create_storage({"type": "s3" | "local", "mode": "async", ...})
  backends/s3.py:      S3-compatible (boto3 / aioboto3)
  backends/local.py:   on-disk filesystem (aiofiles)

Concretely:
  - Drop RustFSObjectStore + common.storage.client (deleted).
  - Drop the RustFS-specific ensure_bucket / presign_put / move_to_trash /
    rewrite_to_public_path / sha256 / put_bytes methods.
  - Migrate backend/storage_api.py + backend/main.py + backend/scripts.py
    + schedule/service.py + schedule/worker.py to the new abstraction.
  - Migrate backend/storage_client.py + schedule/storage_client.py to
    stub status (HTTP wrapper is dead code post-migration; rewrite pending).
  - Rename all RUSTFS_* env vars to S3_* across .env.example,
    docker-compose.yml, default.conf, scripts/nginx-entrypoint.sh,
    common/config.py.
  - Replace hardcoded rclone remote name "rustfs" with "s3" in
    docker-compose.yml + config.py default.
  - Rename "rustfs" SQLAlchemy column comments + table comments to
    provider-neutral wording; StorageObjects.storage_backend enum
    value moves from "rustfs" to "s3" (DB rows with the old value will
    fail the != "s3" check until a one-shot migration is applied).
  - Drop unused common/src/common/migrations/{README,env.py,script.py.mako}
    (the alembic setup lives in /migrations/, not here).

Migration of the old abstractions has been done in one pass; per-route
method calls (delete / stat / put / get_url) are now direct one-liners
against AsyncStorageBackend.

After this commit:
  - All Python imports resolve; routes compile (compileall green).
  - s3 mode is fully wired.
  - Routes that depended on removed methods (presign_put, move_to_trash,
    rewrite_to_public_path, head() metadata) raise NotImplementedError
    with a one-line TODO; rewriting these route handlers is the next step.

.env.example

@@ -24,27 +24,42 @@ JWT_SECRET=change-this-development-secret
 # ============================================================================
 INITIAL_ADMIN_PASSWORD=admin12345
 
-# Object storage (S3-compatible, RustFS).
-# RUSTFS_ENDPOINT is the single upstream URL consumed by all 4 services:
+# Object storage. Two modes are supported:
+#   STORAGE_BACKEND=s3    — connects to an S3-compatible service (MinIO,
+#                           RustFS, SeaweedFS, AWS S3, …). Requires the
+#                           S3_* block below.
+#   STORAGE_BACKEND=local — stores objects on the local filesystem under
+#                           LOCAL_STORAGE_BASE_DIR. Backend and runtime
+#                           share this directory via a Docker volume
+#                           (docker-compose.yml mounts `local-storage`).
+#                           Useful for dev, single-node, air-gapped.
+STORAGE_BACKEND=s3
+LOCAL_STORAGE_BASE_DIR=/data
+
+# Object storage (S3-compatible). Only used when STORAGE_BACKEND=s3.
+# S3_ENDPOINT is the single upstream URL consumed by all 4 services:
 #   - nginx (via scripts/nginx-entrypoint.sh, which parses host + port)
 #   - backend / runtime / schedule (passed through to boto3 / rclone)
-# RUSTFS_ACCESS_KEY / RUSTFS_SECRET_KEY are read by Python code in
+# S3_ACCESS_KEY / S3_SECRET_KEY are read by Python code in
 # backend/ and schedule/ (boto3 credentials).
 #
-# RustFS buckets are purpose-named. Currently we have:
-#   RUSTFS_WORKSPACE_BUCKET — workspace files (notebooks, scripts, working
+# S3 buckets are purpose-named:
+#   S3_WORKSPACE_BUCKET — workspace files (notebooks, scripts, working
 #     copies); layout is ``s3://<bucket>/<workspace_id>/...``.
-# Future: RUSTFS_VERSION_BUCKET, RUSTFS_RUN_LOG_BUCKET, ...
-RUSTFS_HOST=127.0.0.1
-RUSTFS_PORT=9000
-RUSTFS_ENDPOINT=http://127.0.0.1:9000
-RUSTFS_ACCESS_KEY=change-me
-RUSTFS_SECRET_KEY=change-me
-RUSTFS_WORKSPACE_BUCKET=workspaces
-RUSTFS_VERSION_BUCKET=versions
-RUSTFS_RUN_LOG_BUCKET=run-logs
-RUSTFS_TRASH_BUCKET=trash
-RUSTFS_TRASH_RETENTION_DAYS=30
+#   S3_VERSION_BUCKET — immutable script-version artifacts.
+#   S3_RUN_LOG_BUCKET — schedule run logs and execution results.
+#   S3_TRASH_BUCKET — soft-deleted objects; source bucket key is preserved
+#     as a prefix so restore is a same-key move.
+S3_HOST=127.0.0.1
+S3_PORT=9000
+S3_ENDPOINT=http://127.0.0.1:9000
+S3_ACCESS_KEY=change-me
+S3_SECRET_KEY=change-me
+S3_WORKSPACE_BUCKET=workspaces
+S3_VERSION_BUCKET=versions
+S3_RUN_LOG_BUCKET=run-logs
+S3_TRASH_BUCKET=trash
+S3_TRASH_RETENTION_DAYS=30
 
 # rclone RC (HTTP control API). The runtime container starts rclone with
 # `--rc --rc-addr 0.0.0.0:5572 --rc-no-auth` (see runtime/src/runtime/mount.py),