feat: add privacy policy; wire GA4 and Umami at build time

- /privacy/ page covering the contact form (Formspree), Umami, GA4 and
  server logs; linked from the footer
- Analytics.astro: GA4 alongside Umami, both limited to the production
  host so staging and local builds are not counted
- Dockerfile ARGs + CI build args from repo Actions variables
  GA_MEASUREMENT_ID and UMAMI_WEBSITE_ID (non-secret)

.env.example

@@ -1,7 +1,9 @@
 PUBLIC_SITE_URL=https://www.bluecapstrategies.com
 PUBLIC_CONTACT_FORM_ENDPOINT=
 
-# Analytics — side-by-side trial (ADR-0005). Leave blank to disable each.
+# Analytics — leave blank to disable each. Production values come from CI (Dockerfile ARGs).
+# Google Analytics 4:
+PUBLIC_GA_MEASUREMENT_ID=
 # PostHog (Cloud free tier recommended for the trial):
 PUBLIC_POSTHOG_KEY=
 PUBLIC_POSTHOG_HOST=https://us.i.posthog.com