Render the app privacy policies from the app repos

Each policy now has exactly one copy: docs/PRIVACY.md in the app's own
repository. The pages keep their URLs and chrome and render that file
through a content collection, so the published page and the app's own
documentation cannot drift.

scripts/sync-external.mjs shallow-clones both repos into external/ from
prebuild and predev — not from CI: Coolify builds the site from the repo,
so a checkout that only ran in a Gitea job would never reach the deploy.
It falls back to the raw file if git is unavailable, and takes <APP>_REF
or <APP>_LOCAL for work against a branch or an unpushed working copy.

A missing, empty or malformed policy fails the build, verified against the
real image: the deploy stops rather than publishing an empty privacy page.

src/utils/policy.ts

@@ -0,0 +1,21 @@
+import { getCollection } from 'astro:content';
+
+/**
+ * The single entry of an app's privacy-policy collection.
+ *
+ * The Markdown lives in the app's own repository (see scripts/sync-external.mjs);
+ * this site holds no copy. If the checkout is missing or empty the collection
+ * resolves nothing, and a privacy page that renders nothing is worse than a
+ * failed build — so this throws rather than returning undefined.
+ */
+export async function getPolicy(collection: 'calendulaPolicy' | 'agendulaPolicy') {
+  const entries = await getCollection(collection);
+  if (entries.length !== 1) {
+    throw new Error(
+      `[policy] ${collection} resolved ${entries.length} entries, expected exactly 1. ` +
+        'The app repository under external/ is missing, empty, or holds more than ' +
+        'one PRIVACY.md. Run `npm run sync:external` and read its output.'
+    );
+  }
+  return entries[0];
+}