Get organization settings

Per-organization settings migrated from instance-wide env vars. Surfaced in the Pro admin panel and stored in the organization_settings table. Starts with the OIDC account-linking toggles; designed to grow as more settings move org-level.

Each value is tri-state: null means "not set — inherit the instance/env default", while an explicit true/false overrides the env. The fallback to the env default is resolved in the auth layer, not here.