fix(conversations): handle malformed disappearing settings JSON

[aiirvizionz]

Summary

• return a 400 response for malformed per-conversation disappearing-message JSON
• avoid treating client JSON syntax errors as generic internal server errors
• add focused regression coverage that verifies participant lookup/update does not run 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/conversations/[id]/disappearing-messages/route.test.js" (3 tests)

Billing

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