perf(ci): throttle only Docker Hub copies in the daily mirror and raise the limit

The daily mirror waited about 70 s before each of its 195 copies (50 images
per hour), roughly 3.8 h of pauses, and logged no single rate-limit error.
37 of the copies come from ghcr.io, registry.gitlab.com, quay.io,
registry.opencode.de and mcr.microsoft.com, which the Docker Hub pull limit
does not cover, and --only-missing also waited before images it then skipped.
The throttle now waits only before an actual copy from docker.io, and the
schedule allows 150 Docker Hub copies per hour (24 s apart).

Part of #1324

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/unit/python/cli/contributing/mirror/test_sync.py

@@ -7,6 +7,57 @@ import cli.contributing.mirror.sync.__main__ as sync_main
 from cli.contributing.mirror.model import ImageRef
 
 
+class TestThrottle(unittest.TestCase):
+    def test_only_docker_hub_copies_wait_for_the_throttle(self) -> None:
+        def image(name: str, registry: str) -> ImageRef:
+            return ImageRef(
+                role="r",
+                service=name,
+                name=name,
+                version="1",
+                source=f"{registry}/{name}:1",
+                registry=registry,
+            )
+
+        images = [
+            image("a", "docker.io"),
+            image("b", "ghcr.io"),
+            image("c", "quay.io"),
+            image("d", "docker.io"),
+        ]
+        with (
+            patch(
+                "cli.contributing.mirror.sync.__main__.iter_role_images",
+                return_value=images,
+            ),
+            patch(
+                "cli.contributing.mirror.sync.__main__.iter_runner_images",
+                return_value=[],
+            ),
+            patch.object(sync_main.GHCRProvider, "image_base", return_value="g/x"),
+            patch.object(sync_main.GHCRProvider, "mirror") as mock_mirror,
+            patch.object(sync_main.time, "monotonic", return_value=0.0),
+            patch.object(sync_main.time, "sleep") as mock_sleep,
+            patch(
+                "sys.argv",
+                [
+                    "mirror-sync",
+                    "--ghcr-namespace",
+                    "acme",
+                    "--ghcr-repository",
+                    "myrepo",
+                    "--images-per-hour",
+                    "150",
+                ],
+            ),
+        ):
+            result = sync_main.main()
+
+        self.assertEqual(result, 0)
+        self.assertEqual(mock_mirror.call_count, 4)
+        self.assertEqual([call.args for call in mock_sleep.call_args_list], [(24.0,)])
+
+
 class TestMirrorSync(unittest.TestCase):
     def test_only_missing_skips_existing_destination(self) -> None:
         image = ImageRef(