fix(storage,api): usage cost as per-turn delta of the sdk running total, backfill

src/beaver_gateway/storage/__init__.py

@@ -9,12 +9,14 @@ from beaver_gateway.storage.db import (
     Database,
     append_audit,
     append_usage,
+    backfill_usage_deltas,
     create_token,
     list_active_tokens,
     list_audit_records,
     list_tokens,
     revoke_token,
     touch_token,
+    usage_deltas,
 )
 from beaver_gateway.storage.models import (
     AuditLog,
@@ -45,10 +47,12 @@ __all__ = [
     "Usage",
     "append_audit",
     "append_usage",
+    "backfill_usage_deltas",
     "create_token",
     "list_active_tokens",
     "list_audit_records",
     "list_tokens",
     "revoke_token",
     "touch_token",
+    "usage_deltas",
 ]