feat: implement task dependencies with validation and update handling

backend/app/models/__init__.py

@@ -5,6 +5,7 @@ from app.models.board_memory import BoardMemory
 from app.models.board_onboarding import BoardOnboardingSession
 from app.models.boards import Board
 from app.models.gateways import Gateway
+from app.models.task_dependencies import TaskDependency
 from app.models.task_fingerprints import TaskFingerprint
 from app.models.tasks import Task
 from app.models.users import User
@@ -17,6 +18,7 @@ __all__ = [
     "BoardOnboardingSession",
     "Board",
     "Gateway",
+    "TaskDependency",
     "Task",
     "TaskFingerprint",
     "User",

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)

backend/app/models/tasks.py

@@ -5,8 +5,8 @@ from uuid import UUID, uuid4
 
 from sqlmodel import Field
 
-from app.models.tenancy import TenantScoped
 from app.core.time import utcnow
+from app.models.tenancy import TenantScoped
 
 
 class Task(TenantScoped, table=True):