feat(P4.3): wire task depend_on check into pending→open transition via reusable helper

app/api/routers/tasks.py

@@ -16,6 +16,7 @@ from app.models.notification import Notification as NotificationModel
 from app.api.deps import get_current_user_or_apikey
 from app.api.rbac import check_project_role, check_permission, ensure_can_edit_task
 from app.services.activity import log_activity
+from app.services.dependency_check import check_task_deps
 
 router = APIRouter(tags=["Tasks"])
 
@@ -293,7 +294,7 @@ def transition_task(
     # P5.1: enforce state-machine
     _check_transition(old_status, new_status)
 
-    # P5.2: pending -> open requires milestone to be undergoing (dependencies checked later)
+    # P5.2: pending -> open requires milestone to be undergoing + task deps satisfied
     if old_status == "pending" and new_status == "open":
         milestone = db.query(Milestone).filter(Milestone.id == task.milestone_id).first()
         if milestone:
@@ -303,6 +304,13 @@ def transition_task(
                     status_code=400,
                     detail=f"Cannot open task: milestone is '{ms_status}', must be 'undergoing'",
                 )
+        # P4.3: check task-level depend_on
+        dep_result = check_task_deps(db, task.depend_on)
+        if not dep_result.ok:
+            raise HTTPException(
+                status_code=400,
+                detail=f"Cannot open task: {dep_result.reason}",
+            )
 
     # P5.3: open -> undergoing requires assignee AND operator must be the assignee
     if old_status == "open" and new_status == "undergoing":