Inbound webhook from a provider

Unified ingest endpoint for Linear, GitHub, GitHub Actions, and Vercel webhooks. Validates HMAC signature, checks idempotency, and immediately returns 200. All routing/execution is deferred via `after()`. Duplicate deliveries are detected via the gateway's idempotency layer and return `{ ok: true, duplicate: true }`. Providers register at: `POST /api/integrations/{provider}/oauth/authorize`.

curl -X POST "https://example.com/api/webhooks/ingest/linear" \  -H "Content-Type: application/json" \  -d '{}'

{
  "ok": true,
  "eventId": "string",
  "correlationId": "string",
  "duplicate": true
}

{
  "error": "Session not found"
}

{
  "error": "Session not found"
}

{
  "error": "Session not found"
}