Setup Environment Variable Management

Setup environment management for {{PROJECT}}:

Requirements:
1. Environment variable schema validation
2. Type-safe env access in code
3. Secret management (not in code)
4. Environment-specific configs (dev/staging/prod)
5. .env.example with documentation
6. CI/CD env injection
7. Client vs server env variables
8. Runtime validation

Provide:
- Zod schema for env validation
- Type-safe env module
- .env.example with all variables
- Environment config files
- CI/CD env setup
- Validation on startup
- Error messages for missing vars