refactor(common.storage): drop sync storage abstraction and example_usage

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.

backend/src/backend/services/storage.py

@@ -680,7 +680,7 @@ async def create_download_url_payload(
         expires_in=timedelta(seconds=payload.expires_seconds),
     )
     # Public-host rewriting is now nginx's job (location /storage/). In the
-    # future the boto3 client should be built with the public endpoint so
+    # future the S3 client should be built with the public endpoint so
     # generate_presigned_url returns a public URL directly.
     return {
         "data": {