✨(backend) parse YAML metadata

YAML header is processed, to fill Django
templates with the right parametrized values.

Used an external dependency, frontmatter.

src/backend/pyproject.toml

@@ -47,6 +47,7 @@ dependencies = [
     "nested-multipart-parser==1.5.0",
     "psycopg[binary]==3.1.14",
     "PyJWT==2.8.0",
+    "python-frontmatter==1.0.1",
     "requests==2.31.0",
     "sentry-sdk==1.38.0",
     "url-normalize==1.4.3",