Add CLI command to import configuration from file or stdin, providing
an alternative to the env-var-based automatic import on startup.
Features:
- `import-config --config <path>` to import from a mounted file
- `import-config --stdin` to import from piped input (no file mount needed)
- `import-config --dry-run` to validate config without importing
Changes:
- Add app/server/cli/commands/import-config.ts with new command
- Register importConfigCommand in CLI index
- Refactor config-import.ts: extract runImport() and add applyConfigImport()
for direct config object import (used by CLI)
- Update docs with both import methods (env var and CLI examples)
- Add mirrors support for backup schedules (copy to secondary repos)
- Add oneFileSystem flag to prevent crossing filesystem boundaries
- Support autoRemount=false for volumes (defaults to true)
- Support enabled=false for notification destinations (defaults to true)
- Optimize: move early return before dynamic imports in importBackupSchedules
- Update example config and README with new fields documentation
