Add first deploy backup runbook and JSON snapshot scripts

[sigilabcodex]

This PR adds a narrow maintainer-facing backup/retry path for first-deploy rehearsals.

Highlights:

• Adds docs/first-deploy-backup-runbook.md
• Adds JSON runtime backup and guarded restore scripts
• Adds package scripts for JSON backup/restore
• Ignores generated backup snapshots under data/backups/
• Links the backup runbook from README and the first-deploy rehearsal checklist
• Documents hybrid/Postgres backup as a two-part manual process rather than pretending it is solved

Validation:

• npm run skoss:backup:json passed
• npm run skoss:restore:json -- correctly refused without SKOSS_ALLOW_JSON_RESTORE=1
• npm run typecheck passed
• npm run build passed

Out of scope:

• No production-grade backup system
• No scheduled/offsite/encrypted backups
• No user-facing export manager
• No scripted destructive Postgres restore