fix(auth): fail closed on invite quota count errors

[aiirvizionz]

Summary

• return a safe 500 when issued invite usage cannot be counted
• apply the same quota verification guard to quota checks and invite minting
• add regression coverage for the failed count path

Test

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

uGig billable bugfix candidate. Not counted as revenue until accepted, invoiced, and paid.