➕(backend) install pydantic and django-pydantic-field to strengthen API

Super useful for validation when handling unstructured dictionaries.

Follow qbey's recommendation and align with the
suitenumerique/conversation project approach to improve schema
validation and data integrity.

src/backend/pyproject.toml

@@ -39,6 +39,7 @@ dependencies = [
     "django-redis==6.0.0",
     "django-storages[s3]==1.14.6",
     "django-timezone-field>=5.1",
+    "django-pydantic-field==0.5.4",
     "django==5.2.11",
     "djangorestframework==3.16.1",
     "drf_spectacular==0.29.0",
@@ -50,6 +51,7 @@ dependencies = [
     "markdown==3.10.2",
     "nested-multipart-parser==1.6.0",
     "psycopg[binary]==3.3.2",
+    "pydantic==2.12.4",
     "PyJWT==2.11.0",
     "python-frontmatter==1.1.0",
     "requests==2.32.5",