Analytics improvements:
  - Add PostHog identify() on app load to link events to user profiles
  - Add resetIdentity() on logout to unlink future events
  - Disable all analytics (PostHog + Plunk) for Demo plan organizations
    to prevent test data from polluting production analytics

  Plunk subscription tracking:
  - Move registration tracking from backend to frontend using public API key
  - Frontend calls Plunk directly after password registration
  - OIDC registration stores preference in sessionStorage, tracks after redirect
  - Remove  field from RegisterRequest API

  Backend cleanup:
  - Remove subscribed from RegisterRequest and OidcAuthorizeParams
  - Remove subscribed from auth event metadata
  - Simplify email subscriber (no longer handles registration events)
  - Update track_event signature to remove subscribed parameter