Fix hyphen-join: preserve next row + skip valid hyphenations

Two bugs fixed:
- Apply no longer removes the continuation word from the next row.
  "künden" stays in row 31 — only the current row is repaired
  ("ve" → "ver-"). The original line-break layout is preserved.
- Analysis now skips words that already end with "-" when the direct
  join with the next row is a known word (valid hyphenation, not an error).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

klausur-service/backend/tests/test_gutter_repair.py

@@ -310,8 +310,8 @@ class TestApplySuggestions:
         assert result["applied_count"] == 1
         # Current row: "ve" replaced with "ver-"
         assert grid["zones"][0]["cells"][0]["text"] == "ver-"
-        # Next row: "künden" removed, "und" remains
-        assert grid["zones"][0]["cells"][1]["text"] == "und"
+        # Next row: UNCHANGED — "künden" stays in its original row
+        assert grid["zones"][0]["cells"][1]["text"] == "künden und"
 
     def test_apply_nothing_when_no_accepted(self):
         grid = _make_grid([])