feat(backend): add Pydantic schemas

backend/src/schemas/category.py

@@ -0,0 +1,26 @@
+from uuid import UUID
+
+from pydantic import BaseModel
+
+
+class CategoryBase(BaseModel):
+    """Base category schema."""
+
+    name: str
+    slug: str
+    description: str | None = None
+
+
+class CategoryResponse(CategoryBase):
+    """Category response schema."""
+
+    id: UUID
+    question_count: int = 0
+
+    model_config = {"from_attributes": True}
+
+
+class CategoryListResponse(BaseModel):
+    """Response for category list."""
+
+    items: list[CategoryResponse]