#574 release: admin credits rollup + events endpoints (#573)

Summary

Promotes the admin credits dashboard endpoints from test to main. See #573 for full context, contract details, and preview testing results.

• GET /api/admins/credits/rollup — per-account credit usage totals over selected period
• GET /api/admins/credits/events — drilldown of raw usage_events rows for one account

Also includes the lib/admins refactor that promoted getCutoffMs + PERIOD_DAYS out of lib/admins/privy/ into shared lib/admins/.

Test plan

• Preview deployment fully tested via #573 comment — 11 scenarios green
• CI on #573 passed (lint, format, tests)
• Smoke test on prod after deploy

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Summary by cubic

Adds two admin-only credits endpoints to power the dashboard: a per-account rollup and an events drilldown, with period filters and paging. Centralizes period cutoff logic under lib/admins and updates callers.

• New Features

  • GET /api/admins/credits/rollup — per-account totals for the selected period; joined with account name and latest email; returns page, limit, rows, total_count.
  • GET /api/admins/credits/events — raw usage_events for one account and period; returns page, limit, events, total_count.
  • Periods: all | daily | weekly | monthly (default monthly). Deterministic ordering and CORS preflight on both routes.
  • Rollup aggregation fetches all matching events in batches to avoid Supabase row caps.

• Refactors

  • Move getCutoffMs + PERIOD_DAYS to lib/admins/ and unify "all" to return null; update privy/ and slack/ imports.
  • Extract small helpers: aggregateRollupByAccount, enrichRollupPageWithAccountDetails, selectAllUsageEvents, and Supabase wrappers selectUsageEvents + countUsageEvents.