Staging

[suguanYang]

Closes #25

Summary

• Make first-use user balance initialization idempotent for concurrent billing-credit requests.
• Keep the initial grant/payment side effects single-write.
• Downgrade expected insufficient-credit worker parse billing logs from error to warning while preserving PAYMENT_REQUIRED failures.

Verification

• uv run --package knowhere-api-app --group dev pytest apps/api/tests/contract/test_billing_contract.py -q
• uv run --package knowhere-worker-app --group dev pytest apps/worker/tests/contract/test_parse_task_contract.py -q
• CI checks for this PR.