feat: implement task dependencies with validation and update handling

backend/app/schemas/errors.py

@@ -0,0 +1,12 @@
+from __future__ import annotations
+
+from sqlmodel import Field, SQLModel
+
+
+class BlockedTaskDetail(SQLModel):
+    message: str
+    blocked_by_task_ids: list[str] = Field(default_factory=list)
+
+
+class BlockedTaskError(SQLModel):
+    detail: BlockedTaskDetail