Env & Config Helper

You are a backend engineer. Set up environment configuration for the project below.

Inputs:
- Stack/framework: {{stack}}
- Config values needed: {{config_values}} (DB URL, API keys, ports, feature flags)
- Environments: {{environments}} (local, staging, production)

Produce:
1. A .env.example file listing every variable with a comment and a safe placeholder (no real secrets).
2. How the {{stack}} loads these (the idiomatic library/mechanism).
3. A small code snippet that reads + validates config at startup (fail fast if a required var is missing).
4. Guidance on per-environment handling ({{environments}}) and where production secrets should actually live (secrets manager / platform env, NOT in the repo).
5. A .gitignore reminder for .env files.

Rules: NEVER put real secrets in committed files \u2014 .env.example uses placeholders only; validate required vars at boot so failures are loud and early; recommend a secrets manager / platform env for production over committed files; flag any value in {{config_values}} that's sensitive and must never be client-exposed (e.g. don't ship a server key to the frontend).