docs(website): theming phase 5 as built — brand assets and the cached shell

Records Phase 5 of THEMING_AND_NAV.md as landed and documents the new route
and the shell lifecycle in BACKEND_DESIGN.md.

Where the build differed from the design: the upload is one admin-only call
that writes the settings row too (rather than the generic staff upload plus a
PUT, which would leave unreferenced files and let editors change the site's
identity); brand_assets needed a validator of its own because these are the
only settings values written straight into HTML as URLs; the shell cache
carries a TTL as well as explicit invalidation because it is per process; and
the logo went into all six MoonDot surfaces rather than three.

Also notes what was deliberately left alone: the shell's title and description
still come from BRAND_NAME rather than the admin-set site_title, and fixing
that would change the served shell for instances with no brand_assets row —
which is exactly what the phase's acceptance criterion forbids.

Co-Authored-By: Claude <noreply@anthropic.com>

website/api-route-inventory.json

@@ -253,6 +253,10 @@
       "method": "DELETE",
       "path": "/api/v1/admin/settings/:key"
     },
+    {
+      "method": "POST",
+      "path": "/api/v1/admin/settings/brand-asset/:slot"
+    },
     {
       "method": "POST",
       "path": "/api/v1/admin/shard/account"