feat: externe Links auf Erreichbarkeit prüfen (check-ext-links)

Neuer Befehl `python -m scanner check-ext-links`:
- Liest alle externen <a>-Links aus dem letzten Snapshot
- Prüft jeden per HEAD-Request (Fallback auf GET bei HTTP 405)
- Meldet 4xx/5xx und Verbindungsfehler mit Quelladressen
- Report unter reports/<ts>_ext-links/
- Exit-Code 1 bei kaputten Links, 0 wenn alle erreichbar

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_ext_links.py

@@ -0,0 +1,82 @@
+"""Tests for external link reachability check."""
+from unittest.mock import MagicMock, patch
+
+import pytest
+import requests
+
+from scanner.checker import check_external_links
+
+
+def _mock_response(status_code=200, url="https://example.com/"):
+    resp = MagicMock()
+    resp.status_code = status_code
+    resp.url = url
+    return resp
+
+
+class TestCheckExternalLinks:
+    def test_reachable_link_recorded(self):
+        with patch("scanner.checker.requests.Session") as MockSession:
+            session = MockSession.return_value.__enter__.return_value
+            MockSession.return_value = MagicMock()
+            inst = MockSession.return_value
+            inst.head.return_value = _mock_response(200)
+            inst.close = MagicMock()
+
+            results = check_external_links(["https://example.com/"], delay=0)
+
+        assert results["https://example.com/"]["status"] == 200
+        assert results["https://example.com/"]["error"] is None
+
+    def test_broken_link_404_recorded(self):
+        with patch("scanner.checker.requests.Session") as MockSession:
+            inst = MockSession.return_value
+            inst.head.return_value = _mock_response(404, "https://example.com/gone")
+            inst.close = MagicMock()
+
+            results = check_external_links(["https://example.com/gone"], delay=0)
+
+        assert results["https://example.com/gone"]["status"] == 404
+
+    def test_connection_error_recorded(self):
+        with patch("scanner.checker.requests.Session") as MockSession:
+            inst = MockSession.return_value
+            inst.head.side_effect = requests.exceptions.ConnectionError("refused")
+            inst.close = MagicMock()
+
+            results = check_external_links(["https://unreachable.example.com/"], delay=0)
+
+        r = results["https://unreachable.example.com/"]
+        assert r["status"] is None
+        assert "refused" in r["error"]
+
+    def test_head_405_falls_back_to_get(self):
+        with patch("scanner.checker.requests.Session") as MockSession:
+            inst = MockSession.return_value
+            inst.head.return_value = _mock_response(405)
+            get_resp = _mock_response(200)
+            get_resp.close = MagicMock()
+            inst.get.return_value = get_resp
+            inst.close = MagicMock()
+
+            results = check_external_links(["https://example.com/no-head"], delay=0)
+
+        inst.get.assert_called_once()
+        assert results["https://example.com/no-head"]["status"] == 200
+
+    def test_multiple_urls_all_checked(self):
+        urls = [f"https://example.com/{i}" for i in range(3)]
+        with patch("scanner.checker.requests.Session") as MockSession:
+            inst = MockSession.return_value
+            inst.head.return_value = _mock_response(200)
+            inst.close = MagicMock()
+
+            results = check_external_links(urls, delay=0)
+
+        assert len(results) == 3
+        assert inst.head.call_count == 3
+
+    def test_empty_list_returns_empty(self):
+        with patch("scanner.checker.requests.Session"):
+            results = check_external_links([], delay=0)
+        assert results == {}