feat(backend): add Pydantic schemas

backend/src/schemas/common.py

@@ -0,0 +1,18 @@
+from pydantic import BaseModel
+
+
+class PaginationParams(BaseModel):
+    """Pagination parameters."""
+
+    page: int = 1
+    limit: int = 20
+
+
+class PaginatedResponse[T](BaseModel):
+    """Generic paginated response."""
+
+    items: list[T]
+    total: int
+    page: int
+    limit: int
+    pages: int