Better env strategy

[must108]

Background

Currently, we use @t3-oss/env-nextjs for createEnv. It would be better to use an env strategy that works across runtimes, such as @t3-oss/env-core, for some files.

Implementation

Listed below are all the files that use the Next.js version:

• packages/email/src/env.ts *
• packages/api/src/env.ts *
• apps/guild/src/env.ts
• packages/auth/src/env.ts *
• apps/2025/src/env.ts
• packages/db/src/env.ts *
• apps/club/src/env.ts
• apps/blade/src/env.ts
• packages/utils/src/env.ts *

(*) - should be changed, shared/not Next.js