The async-only direction was already the only one used in production:
* create_storage never accepted mode=sync; build_storage_config always
emitted mode=async; zero callers referenced StorageBackend / SyncData
/ S3StorageBackend.sync / LocalStorageBackend.sync anywhere.
* Drop the parallel sync base class, the sync concrete classes in
backends/local.py and backends/s3.py, and the boto3 dependency.
* Drop example_usage.py (zero importers; demonstration code, not part
of the public surface).
* Rename LocalAsyncStorageBackend -> LocalStorageBackend,
S3AsyncStorageBackend -> S3StorageBackend to reflect the single
remaining class per type.
* Tighten create_storage: any mode=... key now raises StorageConfigError
with the new pointer (settings.storage_backend controls behavior).
* Cleanup call sites: schedule.application.service.build_object_store
no longer passes mode=async to create_storage.
* Cosmetic touch-ups in backend/services/storage.py and
common/config.py docstrings where they still said "boto3" instead of
"S3 client".
Public API surface preserved: AsyncStorageBackend / ObjectMeta /
create_storage / build_storage_config / register_backend all keep
their names and call signatures. backend tests: 136 passed.
tao.chen
6050880c98