Add Keycloak auth configuration (env + config loading)

[JulienLouisSchneider]

Issue 1 — Add Keycloak auth configuration (env + config loading)

Description

Add the configuration required for validating Keycloak JWTs in the customer-service. This includes issuer URL and JWKS URL, loaded via the existing config/config.go and exposed to the app.

Checklist

• Add new env vars to .env.example:

  • KEYCLOAK_ISSUER (ex: http://localhost:8888/realms/dev)

  • KEYCLOAK_JWKS_URL (ex: http://localhost:8888/realms/dev/protocol/openid-connect/certs)

  • (optional) KEYCLOAK_AUDIENCE (if you decide to validate aud)

• Extend config/config.go to load these variables.

• Wire config into cmd/main.go so routes/middlewares can access it.

• Document in backend README how to set these values for local Keycloak.

Acceptance Criteria

• App boots with KEYCLOAK_ISSUER + KEYCLOAK_JWKS_URL set.

• App fails fast with a clear error if one of the required vars is missing.