feat: implement task dependencies with validation and update handling

backend/app/models/task_dependencies.py

@@ -0,0 +1,31 @@
+from __future__ import annotations
+
+from datetime import datetime
+from uuid import UUID, uuid4
+
+from sqlalchemy import CheckConstraint, UniqueConstraint
+from sqlmodel import Field
+
+from app.core.time import utcnow
+from app.models.tenancy import TenantScoped
+
+
+class TaskDependency(TenantScoped, table=True):
+    __tablename__ = "task_dependencies"
+    __table_args__ = (
+        UniqueConstraint(
+            "task_id",
+            "depends_on_task_id",
+            name="uq_task_dependencies_task_id_depends_on_task_id",
+        ),
+        CheckConstraint(
+            "task_id <> depends_on_task_id",
+            name="ck_task_dependencies_no_self",
+        ),
+    )
+
+    id: UUID = Field(default_factory=uuid4, primary_key=True)
+    board_id: UUID = Field(foreign_key="boards.id", index=True)
+    task_id: UUID = Field(foreign_key="tasks.id", index=True)
+    depends_on_task_id: UUID = Field(foreign_key="tasks.id", index=True)
+    created_at: datetime = Field(default_factory=utcnow)