fix(auth): handle malformed key backup JSON

[aiirvizionz]

Summary

• return a 400 response for malformed key-backup JSON payloads
• avoid treating client JSON syntax errors as generic internal server errors
• add focused regression coverage that verifies no storage/database work runs for malformed JSON

Validation

• git diff --check
• node node_modules\vitest\vitest.mjs run --config $env:TEMP\qryptchat-vitest-minimal.config.mjs "src/app/api/auth/key-backup/route.test.js" (4 tests)

Billing

• uGig billable PR candidate: expected invoice $0.50 if accepted/merged
• Not counted as revenue until paid/settled