Use Better Auth impersonation and add user bans

[theNEXlevel]

Summary

• Replace custom impersonation endpoints/cookie handling with Better Auth's native admin plugin.
• Add admin user ban/unban actions with a reason and optional expiration using the native datetime control.
• Display ban reason and expiration in the users table.
• Fix session-dependent header hydration and add the required Prisma migration/client import.
• Add tests and Prisma schema validation to pull-request CI.

Validation

• pnpm test (50 tests passed)
• pnpm exec tsc --noEmit
• pnpm lint
• pnpm exec prisma validate
• pnpm build