Feat/configurable restic command timeout - #38

… timeout

The restic command timeout was hardcoded to 3600s in ResticClient, so large
backups were aborted after one hour. Add a `command_timeout` option that
defaults to None (no timeout) and is only enforced when set in the YAML.
- restic_client: resolve per-command timeout from a configured value
  (None = run without timeout); report the effective timeout in errors
- config_manager: add validated `command_timeout` field (positive int or null)
  and wire it through BackupScript into ResticClient
- config.example.yaml: fully document and group all supported options,
  including the new `command_timeout`
- versioning: use pyproject.toml as the single source of truth and derive
  `__version__` via importlib.metadata (drop hardcoded 0.1.0 / 2.0.0)
- add .cursor rule requiring a version bump on every change
- bump version to 0.5.1