Add service backup region commands and MCP tools

[areina]

• Lists, adds and removes the regions a service's backups are copied to (service backup region list/add/remove), preview only, gated behind TIGER_EXPERIMENTAL like the existing backup command.
• Nests the existing service backup command under a group (service backup list) instead of a bare service backup, so region reads as part of the same resource rather than a same-prefixed sibling command.
• MCP tools mirror the CLI: service_backup_list (renamed from service_backups), service_backup_region_list/add/remove.

Example:

> TIGER_EXPERIMENTAL=1 tiger service backup region add <service-id> --region eu-central-1
✅ Backups for service '<service-id>' will now be copied to 'eu-central-1'.

tiger service backup region list <service-id>
┌──────────────┬───────────────────────┐
│   REGION     │         ADDED         │
├──────────────┼───────────────────────┤
│ eu-central-1 │ 2026-09-01 18:14 CEST │
└──────────────┴───────────────────────┘

> TIGER_EXPERIMENTAL=1 tiger service backup region remove <service-id> --region eu-central-1
✅ Backups for service '<service-id>' will no longer be copied to 'eu-central-1'.