Add shapes module: Pydantic API surface for Django models

Imported from separate development branch. Provides Shape, Diff, and
NestedDiff classes for defining typed Pydantic schemas backed by Django
model querysets via django-readers.

Optional dependency: install with djarea[shapes] to get django-readers.
Import is guarded so the rest of djarea works without it.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

django/pyproject.toml

@@ -22,6 +22,9 @@ allauth = [
 webauthn = [
     "fido2>=2.0",
 ]
+shapes = [
+    "django-readers>=2.0",
+]
 dev = [
     "pytest>=8.0",
     "pytest-django>=4.9",