feat(indexer): add event statistics endpoint with type breakdown

[OTimileyin]

Closes #79

Summary

Adds GET /api/events/stats endpoint to the indexer service for retrieving aggregate event counts by type within a configurable time window.

Endpoint

GET /api/events/stats?from=2025-01-01T00:00:00Z&to=2025-06-01T00:00:00Z

Requires x-service-token header (inter-service auth).

Response

{
  "total": 42,
  "byType": {
    "PaymentCompleted": 30,
    "SettlementTriggered": 12
  },
  "timeRange": {
    "from": "2025-01-01T00:00:00.000Z",
    "to": "2025-06-01T00:00:00.000Z"
  }
}

• from / to — optional ISO-8601 dates; to defaults to now, omitted from returns all events up to to
• Invalid or inverted dates → 400 VALIDATION_ERROR
• Missing auth → 401

Implementation notes

• Uses existing DateRangeQuery Zod schema from @bettapay/validation for input validation
• Aggregates via prisma.indexedEvent.groupBy({ by: ['type'], _count: true }) with indexedAt range filter
• Maps Prisma's [{ type, _count }] output into a flat Record<string, number> and sums total server-side

Verification

• pnpm --filter indexer type-check — zero new type errors
• pnpm --filter indexer test — 3 new tests pass (auth rejection, invalid date 400, inverted range 400)
• Pre-existing failures limited to Redis-dependent rate-limit tests (unrelated)

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