Add event counting methods and generic pagination schema

Introduce methods to count user, public, and upcoming events to enhance CRUD functionality for events. Additionally, add a `PaginatedResponse` schema to simplify and standardize paginated API responses. These updates support improved data querying and response handling.

backend/app/schemas/common.py

@@ -0,0 +1,22 @@
+from typing import Generic, List, TypeVar
+from pydantic import BaseModel
+
+T = TypeVar('T')
+
+class PaginatedResponse(BaseModel, Generic[T]):
+    """
+    Generic schema for paginated responses.
+
+    Attributes:
+        total (int): Total number of items
+        items (List[T]): List of items in the current page
+        page (int): Current page number
+        size (int): Number of items per page
+    """
+    total: int
+    items: List[T]
+    page: int
+    size: int
+
+    class Config:
+        from_attributes = True