POST /api/projects/<id>/collections, owner/admin-gated, commits a
.collection.yaml to the content repo main via bot.create_collection, then
re-mirrors the registry so the collections row appears (§22.2). Adds GET
list/one collection routes. Extends FakeGitea to model directory listings so
the mirror's content-repo walk is exercised end to end.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>