feat: add approval-task links model and related functionality for task associations

backend/app/models/__init__.py

@@ -2,6 +2,7 @@
 
 from app.models.activity_events import ActivityEvent
 from app.models.agents import Agent
+from app.models.approval_task_links import ApprovalTaskLink
 from app.models.approvals import Approval
 from app.models.board_group_memory import BoardGroupMemory
 from app.models.board_groups import BoardGroup
@@ -22,6 +23,7 @@ from app.models.users import User
 __all__ = [
     "ActivityEvent",
     "Agent",
+    "ApprovalTaskLink",
     "Approval",
     "BoardGroupMemory",
     "BoardMemory",