Strengthen reusable Astro site foundation

docs/content-model.md

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+# Static site content model
+
+Each hosted site has a validated `sites/<key>/site.json`. It owns its brand
+fallback/logo, SEO image, navigation, page content, contact information, and
+whether a physical-address map is displayed. Shared Astro components render
+that data without a per-site code fork.
+
+Current reusable primitives include the base SEO layout, header/footer,
+full-page hero, section heading, CTA band, review cards, and optional location
+map. A real logo can be provided as `/images/<safe-name>.svg` or image asset;
+until then the configured short mark and shared favicon are used.
+
+Images must live under `public/images/`, be referenced with descriptive alt
+text where rendered, and remain at or below the 500 KiB pilot limit. The build
+will reject malformed site JSON and oversize source images.