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

app/services/dependency_check.py

@@ -111,3 +111,38 @@ def check_milestone_deps(
             result.blockers.append(f"Dependent tasks not {required_status}: {incomplete_tasks}")
 
     return result
+
+
+def check_task_deps(
+    db: Session,
+    depend_on: str | None,
+    *,
+    required_status: str = "completed",
+) -> DepCheckResult:
+    """Check whether a task's depend_on tasks are all satisfied.
+
+    Parameters
+    ----------
+    db:
+        Active DB session.
+    depend_on:
+        JSON-encoded list of task IDs (from the task's ``depend_on`` field).
+    required_status:
+        The status dependees must have reached (default ``"completed"``).
+
+    Returns
+    -------
+    DepCheckResult with ``ok=True`` if all deps satisfied, else ``ok=False``
+    with human-readable ``blockers``.
+    """
+    result = DepCheckResult()
+    task_ids = _parse_json_ids(depend_on)
+    if task_ids:
+        dep_tasks: Sequence[Task] = (
+            db.query(Task).filter(Task.id.in_(task_ids)).all()
+        )
+        incomplete = [t.id for t in dep_tasks if _task_status(t) != required_status]
+        if incomplete:
+            result.ok = False
+            result.blockers.append(f"Dependent tasks not {required_status}: {incomplete}")
+    return result