Vercel Sandbox snapshots now allow custom retention periods

Vercel Sandbox snapshots now support configurable expiration periods instead of the previous fixed 7-day limit.

• •Expiration can be set anywhere from 1 day to infinity (0 = never expires)
• •Default snapshot expiration is now 30 days if not explicitly configured
• •Expiration is configurable via SDK: `sandbox.snapshot({ expiration: ms('14d') })`
• •CLI support: `sandbox snapshot sb_1234567890 --stop --expiration 14d`