feat(audit): Authentik events ingest worker (Phase 2 chunk 1)

Background worker that pulls Authentik's /api/v3/events/events/ on a
60s cadence and writes each event into our audit log via AuditService.
External system events now share the same /audit timeline as
internally-recorded platform mutations — operator queries don't have
to cross-reference Authentik's own UI to see logins, password changes,
group membership, impersonation, etc.

Pieces:
- src/schemas/ingest-cursor.schema.ts: one row per source, tracks
  lastEventAt + lastEventId so restarts resume without re-pulling.
- src/schemas/audit-event.schema.ts: new `externalId` field; new
  compound unique index on (source, externalId) with a partial filter
  on externalId being a string. Partial (not sparse) so internally-
  recorded events with externalId=null don't collide.
- src/audit/audit.service.ts: AuditRecordInput grows `externalId` +
  `at` fields. record() now silently swallows MongoError code 11000
  (duplicate key) so re-pulling the cursor overlap doesn't log noise.
- src/integrations/authentik.client.ts: listEvents(since, page,
  pageSize) on the existing client — reuses the admin token and base
  URL the provisioning code already configured.
- src/ingest/action-map.ts: 16 known Authentik actions → dotted
  authentik.* verbs (login, login_failed, password_changed,
  impersonation_started, …). Unknown actions fall through to
  authentik.<raw> rather than getting silently dropped.
- src/ingest/authentik.ingest.ts: OnApplicationBootstrap worker.
  Reads cursor → pulls events with created__gt=cursor, ordering=created
  ASC → paginates forward (10 pages × 100/page safety cap per tick) →
  writes each event with source='authentik' + externalId=pk + at=
  evt.created → advances cursor to the newest seen. inFlight guard
  prevents overlapping ticks. AUDIT_INGEST_ENABLED=false disables for
  test environments.
- Tenant inference: from the user's groups (same convention the portal
  flag-eval proxy uses). Admin groups stripped; first match against a
  real Tenant.slug wins. Unmatched → tenantSlug undefined, event still
  lands in the global timeline.

Smoke-tested: fresh Mongo + restart → 78 Authentik events ingested,
0 duplicates. Performed a login at app.dezky.local → next 60s tick
captured the new login row with actor email + IP. Compound unique
index on (source, externalId) verified to reject re-pulled events
silently (no error logs).

Out of scope here (covered by chunks 2 + 3):
- Stalwart webhook ingest
- OCIS file-tail ingest

services/platform-api/src/app.module.ts

@@ -5,6 +5,7 @@ import { AuditModule } from './audit/audit.module.js'
 import { AuthModule } from './auth/auth.module.js'
 import { FlagsModule } from './flags/flags.module.js'
 import { HealthModule } from './health/health.module.js'
+import { IngestModule } from './ingest/ingest.module.js'
 import { PartnersModule } from './partners/partners.module.js'
 import { SeedModule } from './seed/seed.module.js'
 import { SubscriptionsModule } from './subscriptions/subscriptions.module.js'
@@ -25,6 +26,7 @@ import { UsersModule } from './users/users.module.js'
     UsersModule,
     SubscriptionsModule,
     FlagsModule,
+    IngestModule,
     SeedModule,
   ],
 })