fix(docs): make the build self-contained for Vercel

Vercel's "Root Directory" setting limits the upload to docs/api/, so
the build cannot read sibling paths like ../../server/openapi.json or
../api-content/. A small prebuild script copies both into a vendored
folder inside the project tree (gitignored).

## Changes
- Add scripts/sync-sources.mjs to copy /docs/api-content/ and
  /server/openapi.json into docs/api/src/vendored/ before dev/build
- Wire predev/prebuild npm scripts so the sync runs automatically
- Update astro.config.mjs and src/content.config.ts to read from the
  vendored paths
- Gitignore src/vendored/ — contributors still edit the originals

docs/api/package.json

@@ -5,6 +5,8 @@
   "private": true,
   "description": "Public API documentation for Checkmate. Built with Astro Starlight.",
   "scripts": {
+    "prebuild": "node ./scripts/sync-sources.mjs",
+    "predev": "node ./scripts/sync-sources.mjs",
     "dev": "astro dev",
     "start": "astro dev",
     "build": "astro build",