Improve legacy CloudVPS backup state and reconciliation

[adinvadim]

Context

The documented CloudVPS v1 backup switch is already exposed as:

regru vps backup enable <server-id>
regru vps backup disable <server-id>
regru vps backup restore <server-id> <backup-image-id>

GET /v1/reglets/{server-id} exposes backups_enabled and nullable
last_backup_date. The current CLI preserves those fields in JSON, but it has
no focused status command, normal human/plain vps get output does not show
them, and enable/disable reports only the provider action rather than the
verified postcondition.

Provider sources:

• https://developers.cloudvps.reg.ru/reglets/switch_backups.html
• https://developers.cloudvps.reg.ru/reglets/info.html
• https://api.cloudvps.reg.ru/v1/openapi.json
• https://reg.cloud/support/cloud/oblachnyye-servery/rabota-s-serverom/bekapy-i-snepshoty-na-oblachnykh-serverakh

Scope

• Add regru vps backup status <server-id>.
• Return the normalized enabled state and nullable last-backup date in human,
  plain, and JSON output.
• Surface the same state in vps get without breaking the stable plain-output
  contract; if extending plain output would be breaking, document and retain
  backup status as the stable machine-readable path.
• After an accepted enable/disable action reaches a terminal success state,
  read the server once and verify that backups_enabled matches the requested
  state. Never replay an ambiguously delivered mutation automatically.
• Preserve support for provider boolean variants (true/false, 0/1, and
  the documented legacy string forms).
• Make disable confirmation explain that REG.RU currently retains copies from
  the non-configurable backup for only three calendar days. Do not hard-code a
  price.
• Add command, provider, reconciliation, output, and contract-drift tests.
• Update docs/cloudvps.md, docs/cli-contract.md, and the changelog.

Acceptance criteria

• backup status works without sending a mutation and has stable JSON/plain
  output.
• Successful enable/disable waits are followed by a verified provider state.
• A mismatched or undecodable postcondition fails closed instead of reporting
  success.
• --dry-run, confirmation, --force, --no-input, --no-wait, timeout,
  and outcome_unknown behavior remain consistent with the existing CLI
  contract.
• make check passes.

Boundary

This ticket covers only the published, non-configurable CloudVPS backup
switch. It must not model the newer named backup service with configurable
schedule and retention policies.