fix: frame-extract 默认裁切区域改为画面底部 1/4

- 默认 crop 由 [0,0.82,1,0.18] 调整为 [0,0.75,1,0.25]：字幕很少出现在
  画面上半部分，扩大裁切高度提升 OCR 召回
- 同步更新 ocr-subtitle 工作流 DAG 与参数覆盖测试

tests/test_apps_api.py

@@ -293,12 +293,12 @@ def test_create_run_with_param_overrides() -> None:
         uploaded = client.post(
             f"/api/apps/{workflow_id}/runs",
             files={"file": ("sample.txt", b"x", "text/plain")},
-            data={"params": '{"step": {"crop": [0, 0.82, 1, 0.18]}}'},
+            data={"params": '{"step": {"crop": [0, 0.75, 1, 0.25]}}'},
         )
         assert uploaded.status_code == 200
         run_id = uploaded.json()["id"]
         run = client.get(f"/api/runs/{run_id}").json()
-        assert run["param_overrides"] == {"step": {"crop": [0, 0.82, 1, 0.18]}}
+        assert run["param_overrides"] == {"step": {"crop": [0, 0.75, 1, 0.25]}}
         # 非法 JSON 返回 422。
         bad = client.post(
             f"/api/apps/{workflow_id}/runs",