storage: add local filesystem backend option (STORAGE_BACKEND toggle)

The factory now picks between two backends based on
settings.storage_backend ("s3" default, "local" for dev / single-node /
air-gapped deployments). The new factory helper build_storage_config()
takes one of the 4 PURPOSE_BUCKETS ("workspace" | "version" |
"run_log" | "trash") and returns the kwargs for create_storage(...).

  s3   mode: AsyncStorageBackend over an S3-compatible service
           (S3_WORKSPACE_BUCKET etc. as separate buckets).
  local mode: AsyncStorageBackend over on-disk files; the 4 buckets
           become subdirectories of LOCAL_STORAGE_BASE_DIR (default
           "/data"), so the same 4-bucket layout works in both modes.

Concretely:
  - common/config.py: add storage_backend (default "s3") +
    local_storage_base_dir (default "/data").
  - common/storage/factory.py: add PURPOSE_BUCKETS constant +
    build_storage_config(bucket_name) helper.
  - backend/main.py + backend/storage_api.py: lifespan collapses the
    4-instance construction into one dict comprehension:
      app.state.object_stores = {
        name: create_storage(build_storage_config(name))
        for name in PURPOSE_BUCKETS
      }
    (was 4x ~10-line dicts, one per bucket).
  - runtime/mount.py: when STORAGE_BACKEND=local, skip the rclone mount
    entirely (the shared docker volume at LOCAL_STORAGE_BASE_DIR is the
    store; runtime reads directly).
  - docker-compose.yml: mount the shared local-storage volume at /data
    in both backend and runtime containers.
  - .env.example: document STORAGE_BACKEND + LOCAL_STORAGE_BASE_DIR.

Dependencies added to support both backends:
  - aiofiles>=25.1.0 (local async I/O) to backend + common + runtime.
  - aioboto3>=15.5.0 (async S3) to common.
  - uv.lock regenerated.

After this commit, both modes deploy end-to-end. The s3 mode is the
production default; local mode is opt-in via STORAGE_BACKEND=local.

common/pyproject.toml

@@ -12,6 +12,8 @@ dependencies = [
     "pydantic-settings>=2.14.2",
     "passlib==1.7.4",
     "bcrypt>=4.0,<4.1",
+    "aiofiles>=25.1.0",
+    "aioboto3>=15.5.0",
 ]
 
 [build-system]