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.

.gitignore

@@ -12,3 +12,6 @@ node_modules/
 *.log
 # local screenshots
 .shots/
+
+# App repositories fetched at build time (see scripts/sync-external.mjs).
+external/