[Enhancement] Add resets_on field to Admin API billing response

[daskuntal75]

Summary

The Admin API billing endpoint returns current spend and usage metrics but omits the billing period reset date (resets_on). This date is visible in the console UI but inaccessible via API, forcing developers to hardcode it — which breaks every billing cycle.

Current behaviour

GET /v1/organizations/{org_id}/usage returns:

{
  "current_spend": 0.82,
  "customer_limit": 150.00,
  "tier_name": "Tier 2"
}

No resets_on field.

Expected behaviour

{
  "current_spend": 0.82,
  "customer_limit": 150.00,
  "tier_name": "Tier 2",
  "resets_on": "2026-06-01"
}

Why this matters

Any automation computing normalized spend or scheduling budget alerts must hardcode the reset date:

# Current forced workaround — breaks every month
RESET_DATE = "2026-06-01"  # must be manually updated
days_elapsed = (today - date.fromisoformat(RESET_DATE)).days
normalized_spend = current_spend / (days_elapsed / 30)

This breaks silently at the start of each new billing period and requires manual intervention.

Effort estimate

This field is already computed and displayed in the console UI. Exposing it via the API response appears to be a one-field addition to the serializer.

Reference

Open-source tooling impacted: https://github.com/daskuntal75/llm-cost-kit
Full issue context: https://github.com/daskuntal75/llm-cost-kit/blob/main/docs/anthropic-issues-submit-ready.md