Add Metrics API Endpoints to Event Manager

Create aggregation endpoints in Event Manager API to support integration metrics queries.

• Integration metrics: GET /api/integrations/{source_id}/metrics?org_id={org_id}&days={days}
• Organization metrics: GET /api/organizations/{org_id}/metrics?days={days}
• Time-series: GET /api/organizations/{org_id}/metrics/timeseries?days={days}&bucket={bucket}
• Add database indexes on: (organization_id, timestamp), (source_id, result, timestamp)
• Use SQL aggregation functions (COUNT, GROUP BY) for performance
• Cache responses for 5-15 minutes; limit time range to max 90 days

Example:

Integration metrics response:

{
  "sourceId": "adp-sync",
  "totalEvents": 1250,
  "successCount": 1228,
  "failureCount": 22,
  "successRate": 98.24,
  "lastEventTimestamp": "2025-11-06T15:30:00Z"
}

Organization metrics response:

{
  "integrations": [
    {
      "sourceId": "adp-sync",
      "totalEvents": 1250,
      "successRate": 98.24
    },
    {
      "sourceId": "qb-import",
      "totalEvents": 45,
      "successRate": 88.89
    }
  ],
  "aggregateTotals": {
    "totalEvents": 1295,
    "successCount": 1268,
    "failureCount": 27,
    "successRate": 97.91
  }
}

Time-series response:

{
  "buckets": [
    {
      "timestamp": "2025-11-01T00:00:00Z",
      "totalEvents": 125,
      "successCount": 122,
      "failureCount": 3,
      "integrations": {
        "adp-sync": 100,
        "qb-import": 25
      }
    }
  ]
}