docs(website): theming phases 3-4 as built

Records where the build diverged from the design and why.

- The presets do not live in theme.css as [data-theme] blocks. Section 6.2 is
  marked superseded and a "Phases 3-4 as landed" section explains the inline
  --accent precedence problem that forced server-side resolution.
- Section 4.5's accentInt fix is struck through: getPublic() never exposed
  accentInt, and Discord embeds are colored by a separate process reading env,
  so there was nothing per-request to recompute. Replaced with what was
  actually done -- the bot fetching the effective accent.
- Phase 9 re-scoped. Applying Fantasy on a live instance showed the section 4.8
  rgba literals carry a hue, not just a light/dark assumption, so the dark
  presets need that promotion too.
- BACKEND_DESIGN.md: the new /settings/theme/options route, the `theme` block
  on /public/settings, effective values in the brand block, theme_visual
  validation on PUT /admin/settings, route count 225 -> 226.
- api-route-inventory.json regenerated from the manifest.

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

website/api-route-inventory.json

@@ -900,6 +900,10 @@
     {
       "method": "GET",
       "path": "/api/v1/settings/nav"
+    },
+    {
+      "method": "GET",
+      "path": "/api/v1/settings/theme/options"
     }
   ],
   "internal": [