feat(models): add UserIntegration model, encrypt IMAP passwords, add integrations API

- Add IntegrationDirection/IntegrationType constants and UserIntegration SQLAlchemy model
  with owner_id, direction, integration_type, name, config (JSON), credentials (encrypted),
  is_active, last_used_at, last_error, created_at, updated_at fields
- Fix UserImapAccount password encryption: encrypt_value() on create/update,
  decrypt_value() in IMAP tasks and test-connection endpoint (backward compatible)
- Add Alembic migration 023_add_user_integrations
- Add app/api/integrations.py: CRUD + credentials endpoint with owner-scoped access
- Register integrations router in app/api/__init__.py
- Add 39 tests in tests/test_api_integrations.py covering model, CRUD, encryption

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

app/api/__init__.py

@@ -17,6 +17,7 @@ from app.api.duplicates import router as duplicates_router
 from app.api.files import router as files_router
 from app.api.google_drive import router as google_drive_router
 from app.api.imap_accounts import router as imap_accounts_router
+from app.api.integrations import router as integrations_router
 from app.api.logs import router as logs_router
 from app.api.onboarding import router as onboarding_router
 from app.api.onedrive import router as onedrive_router
@@ -70,3 +71,4 @@ router.include_router(onboarding_router)
 router.include_router(billing_router)
 router.include_router(pipelines_router)
 router.include_router(imap_accounts_router)
+router.include_router(integrations_router)