fix: always report broken internal links + show source pages

Previously, broken links that were part of the baseline ("known broken")
disappeared from the terminal output (score=0, no Klartext mention).

- differ.py: build reverse link index to record which pages link to
  each broken URL; "pages" field added to new_broken/known_broken entries
- plain.py: klartext_befunde() now includes known_broken_urls alongside
  new_broken_urls — broken links are always reported until fixed
- __main__.py: report shows "gefunden auf: <page>" for every broken entry,
  known entries labelled "[bekannt, noch nicht behoben]"
- 3 new tests (169 total)

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

tests/test_differ.py

@@ -234,6 +234,20 @@ class TestBrokenLinks:
         assert len(result["new_broken_urls"]) == 1
         assert result["new_broken_urls"][0]["url"] == "https://b.info/missing/"
         assert result["new_broken_urls"][0]["status"] == 404
+        assert "pages" in result["new_broken_urls"][0]
+
+    def test_broken_url_includes_source_pages(self):
+        """broken entry lists the pages that link to the broken URL."""
+        home = self._make_page("https://b.info/")
+        home["links"]["a"] = [{"url": "https://b.info/missing/", "class": "internal"}]
+        baseline = {"https://b.info/": self._make_page("https://b.info/")}
+        current = {
+            "https://b.info/": home,
+            "https://b.info/missing/": self._make_page("https://b.info/missing/", status=404),
+        }
+        result = compare_snapshots(baseline, current, BASE_CFG)
+        entry = result["new_broken_urls"][0]
+        assert "https://b.info/" in entry["pages"]
 
     def test_known_broken_not_in_new(self):
         broken_page = {"https://b.info/old/": self._make_page("https://b.info/old/", status=404)}
@@ -242,6 +256,7 @@ class TestBrokenLinks:
         result = compare_snapshots(baseline, current, BASE_CFG)
         assert result["new_broken_urls"] == []
         assert len(result["known_broken_urls"]) == 1
+        assert "pages" in result["known_broken_urls"][0]
 
     def test_broken_url_scores(self):
         result = score_diff({