fix(auth): validate key backup export shape

[aiirvizionz]

Summary

• reject encrypted key backup payloads whose JSON string does not decode to an object export
• prevent primitive or array JSON strings from being stored as key backups
• add regression coverage for malformed backup export shape

Test

• node node_modules\vitest\vitest.mjs run --config %TEMP%\qryptchat-vitest-minimal.config.mjs "src/app/api/auth/key-backup/route.test.js"
• git diff --check

uGig billing

• Expected invoice: 0.50 USD if accepted/merged
• Counted as revenue now: no, pending review/payment