feat(notification-ms): add company-level fallback for merchant notification settings

refactor(notification-ms): enhance structured logging across all channels

refactor(notification-ms): migrate config to pydantic settings

chore(notification-ms): enhance config documentation and organization

refactor(notification-ms): remove template CRUD and JWT config from notification service

refactor(notification-ms): migrate templates to company-level and integrate merchant context

feat: notification microservice with per-merchant channel credentials