fix: initialize missing user balance during tier lookup

[suguanYang]

Summary

• initialize missing first-use user billing state from TierService.get_tier by reusing CreditsService.ensure_user_initialized
• preserve the fast cache/existing-balance path and cache the initialized tier after repair
• add a contract regression for API-key users with no user_balances row

Verification

• uv run pytest apps/api/tests/contract/test_billing_contract.py::test_should_initialize_missing_user_balance_during_tier_lookup
• uv run pytest apps/api/tests/contract/test_billing_contract.py
• uv run pytest apps/api/tests/contract/test_api_key_contract.py
• uv run pytest apps/api/tests/contract/test_job_creation_contract.py::test_should_create_a_waiting_file_job_for_an_authenticated_developer
• uv run ruff check apps/api/app/services/rate_limit/tier_service.py apps/api/tests/contract/test_billing_contract.py packages/shared-python/shared/services/billing/credits_service.py
• uv run pyright apps/api/app/services/rate_limit/tier_service.py apps/api/tests/contract/test_billing_contract.py packages/shared-python/shared/services/billing/credits_service.py