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_plain.py

@@ -70,6 +70,20 @@ class TestKlartextBefunde:
         assert any("funktionieren" in s.lower() or "website" in s.lower() for s in saetze)
         _no_jargon(saetze)
 
+    def test_new_broken_internal_link_plain(self):
+        diff = {"new_broken_urls": [{"url": "https://x.de/gone", "status": 404, "pages": []}]}
+        saetze = klartext_befunde(diff)
+        assert any("leere" in s.lower() or "gefunden" in s.lower() for s in saetze)
+        _no_jargon(saetze)
+
+    def test_known_broken_link_still_reported(self):
+        """known_broken_urls must appear in Klartext even with score=0."""
+        diff = {"known_broken_urls": [{"url": "https://x.de/old", "status": 404, "pages": []}]}
+        saetze = klartext_befunde(diff)
+        assert any("leere" in s.lower() or "gefunden" in s.lower() for s in saetze), \
+            "known_broken_urls müssen auch bei Score=0 im Klartext erscheinen"
+        _no_jargon(saetze)
+
 
 class TestWasTun:
     def test_red_has_urgent_steps(self):