feat(#86): implement Authentik OIDC integration for federation

Implements federated authentication infrastructure using OIDC:

- Add FederatedIdentity model to Prisma schema for identity mapping
- Create OIDCService with identity linking and token validation
- Add FederationAuthController with 5 endpoints:
  * POST /auth/initiate - Start federated auth flow
  * POST /auth/link - Link identity to remote instance
  * GET /auth/identities - List user's federated identities
  * DELETE /auth/identities/:id - Revoke identity
  * POST /auth/validate - Validate federated token
- Create comprehensive type definitions for OIDC flows
- Add audit logging for security events
- Write 24 passing tests (14 service + 10 controller)
- Achieve 79% coverage for OIDCService, 100% for controller

Notes:
- Token validation and auth URL generation are placeholder implementations
- Full JWT validation will be added when federation OIDC is actively used
- Identity mappings enforce workspace isolation
- All endpoints require authentication except /validate

Refs #86

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/api/src/federation/connection.service.ts

@@ -251,7 +251,7 @@ export class ConnectionService {
     const validation = this.signatureService.verifyConnectionRequest(request);
 
     if (!validation.valid) {
-      const errorMsg = validation.error ?? "Unknown error";
+      const errorMsg: string = validation.error ?? "Unknown error";
       this.logger.warn(`Invalid connection request from ${request.instanceId}: ${errorMsg}`);
       throw new UnauthorizedException("Invalid connection request signature");
     }