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",

backend/app/models/approval_task_links.py

@@ -0,0 +1,32 @@
+"""Approval-task link model for many-to-many approval associations."""
+
+from __future__ import annotations
+
+from datetime import datetime
+from uuid import UUID, uuid4
+
+from sqlalchemy import UniqueConstraint
+from sqlmodel import Field
+
+from app.core.time import utcnow
+from app.models.base import QueryModel
+
+RUNTIME_ANNOTATION_TYPES = (datetime,)
+
+
+class ApprovalTaskLink(QueryModel, table=True):
+    """Map an approval request to one task (many links per approval allowed)."""
+
+    __tablename__ = "approval_task_links"  # pyright: ignore[reportAssignmentType]
+    __table_args__ = (
+        UniqueConstraint(
+            "approval_id",
+            "task_id",
+            name="uq_approval_task_links_approval_id_task_id",
+        ),
+    )
+
+    id: UUID = Field(default_factory=uuid4, primary_key=True)
+    approval_id: UUID = Field(foreign_key="approvals.id", index=True)
+    task_id: UUID = Field(foreign_key="tasks.id", index=True)
+    created_at: datetime = Field(default_factory=utcnow)