auth

Shared authentication primitives for Shiny backend services.

• UserMiddleware(signingKey) — verifies the HMAC-signed user header the gateway propagates (ADR-0005) and injects the *User into the request context.
• FromContext(ctx) / User.HasRole(...) — read the authenticated user.
• MissingDeployedSecrets(env, secrets) — startup guard that fails closed when required secrets are empty in staging/production (ADR-0005/0006).

Replaces the byte-identical auth package and secrets_guard.go previously copied into every service.