fix(api-keys): backfill new limit usage

[mgwals]

Summary

• Backfill newly added API key limits from successful request-log usage in the active lookback window.
• Preserve existing matching limit counters and keep resetUsage as the explicit zero-reset path.
• Add OpenSpec coverage and regression tests for total-token, model-filtered, reset, and non-success-log behavior.

Closes #518

Validation

• openspec validate --specs
• openspec validate backfill-api-key-limit-current-usage
• uv run pytest -q tests/integration/test_api_keys_trends_api.py::test_adding_limit_backfills_current_window_usage tests/integration/test_api_keys_trends_api.py::test_adding_model_scoped_limit_backfills_only_matching_model tests/integration/test_api_keys_trends_api.py::test_adding_limit_with_reset_usage_keeps_current_value_zero
• uv run pytest -q tests/unit/test_api_keys_service.py tests/integration/test_api_keys_trends_api.py
• uv run ruff check app/modules/api_keys tests/integration/test_api_keys_trends_api.py tests/unit/test_api_keys_service.py
• uv run ruff format --check app/modules/api_keys tests/integration/test_api_keys_trends_api.py tests/unit/test_api_keys_service.py
• uv run ty check
• codex-review-high --base origin/main