feat(admin): export/import configuration bundle for migration

Backend: GET/POST /api/v1/admin/export-config and /import-config
  Export includes (sensitive values redacted):
  - system_configs (email/jira settings)
  - webhook_configs (secrets redacted)
  - sso_configs (private key redacted)
  - scoring_config (weights)
  - test_templates (source=custom only)
  - users (no passwords/tokens, must_change_password=True on import)
  Import is idempotent — upsert by natural keys, safe to run multiple times.

Frontend: ExportImportSection in SystemPage (admin only)
  - 'Export Configuration' → downloads aegis-config-YYYY-MM-DD.json
  - 'Import Configuration' → file picker, sends JSON, shows summary
  - Visual checklist of what is/isn't included in the export

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/main.py

@@ -40,6 +40,7 @@ from app.routers import osint as osint_router
 from app.routers import webhooks as webhooks_router
 from app.routers import detection_lifecycle as detection_lifecycle_router
 from app.routers import intel as intel_router
+from app.routers import admin_config as admin_config_router
 from app.routers import ownership as ownership_router
 from app.routers import attack_paths as attack_paths_router
 from app.routers import knowledge as knowledge_router
@@ -165,6 +166,7 @@ app.include_router(scores_router.router, prefix="/api/v1")
 app.include_router(operational_metrics_router.router, prefix="/api/v1")
 app.include_router(compliance_router.router, prefix="/api/v1")
 app.include_router(intel_router.router, prefix="/api/v1")
+app.include_router(admin_config_router.router, prefix="/api/v1")
 app.include_router(snapshots_router.router, prefix="/api/v1")
 app.include_router(jira_router.router, prefix="/api/v1")
 app.include_router(worklogs_router.router, prefix="/api/v1")