Fix restore drill for retained forensic database snapshots

[Phloraxx]

Summary

• validate only the root SQLite databases that PocketBase would actually boot after restore
• ignore nested quarantine/manual-safety forensic .db snapshots
• add regression coverage with a deliberately corrupt nested snapshot

Production finding

The active data.db and rebuilt auxiliary.db are clean. The existing v1 restore drill false-fails on a retained historical quarantine database from the August 11 incident.

Validation

• go test ./internal/backups -count=1
• go vet ./internal/backups
• staticcheck ./internal/backups
• patched v2 binary passed restore drill against the real post-repair production backup (2 active databases)

No payment/evidence/refund behavior changes.