Harden Convex Clerk issuer matching

[codybmenefee]

Summary

• normalize Clerk issuer domain in convex auth config (strip trailing slash)
• add safe fallbacks when CLERK_JWT_ISSUER_DOMAIN is unset
• register both issuer variants (with and without trailing slash) for custom JWT provider compatibility

Why
Prod still shows intermittent Convex auth rejection: "No auth provider found matching the given token". This can happen when issuer formatting differs (trailing slash) or env is missing/mis-set.

Validation

• cd app && npx eslint convex/auth.config.ts
• cd app && pnpm run build