Fix gutter repair: detect short fragments + show spell alternatives

- Lower min word length from 3→2 for hyphen-join candidates so fragments
  like "ve" (from "ver-künden") are no longer skipped
- Return all spellchecker candidates instead of just top-1, so user can
  pick the correct form (e.g. "stammeln" vs "stammelt")
- Frontend shows clickable alternative buttons for spell_fix suggestions
- Backend accepts text_overrides in apply endpoint for user-selected alternatives

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

klausur-service/backend/tests/test_gutter_repair.py

@@ -13,6 +13,8 @@ from cv_gutter_repair import (
     _try_spell_fix,
     _edit_distance,
     _word_is_at_gutter_edge,
+    _MIN_WORD_LEN_SPELL,
+    _MIN_WORD_LEN_HYPHEN,
     analyse_grid_for_gutter_repair,
     apply_gutter_suggestions,
 )
@@ -127,27 +129,33 @@ class TestTryHyphenJoin:
 
 @needs_spellchecker
 class TestTrySpellFix:
-    def test_fix_garbled_ending(self):
-        # "stammeli" should suggest "stammeln"
+    def test_fix_garbled_ending_returns_alternatives(self):
+        # "stammeli" should return a correction with alternatives
         result = _try_spell_fix("stammeli", col_type="column_de")
         assert result is not None
-        corrected, conf = result
-        assert corrected == "stammeln"
+        corrected, conf, alts = result
+        # The best correction is one of the valid forms
+        all_options = [corrected] + alts
+        all_lower = [w.lower() for w in all_options]
+        # "stammeln" must be among the candidates
+        assert "stammeln" in all_lower, f"Expected 'stammeln' in {all_options}"
 
     def test_known_word_not_fixed(self):
         # "Haus" is correct — no fix needed
         result = _try_spell_fix("Haus", col_type="column_de")
         # Should be None since the word is correct
-        # (unless spellchecker suggests something else)
-        # Either None or same word is acceptable
         if result is not None:
-            corrected, _ = result
+            corrected, _, _ = result
             assert corrected.lower() == "haus"
 
     def test_short_word_skipped(self):
         result = _try_spell_fix("ab")
         assert result is None
 
+    def test_min_word_len_thresholds(self):
+        assert _MIN_WORD_LEN_HYPHEN == 2
+        assert _MIN_WORD_LEN_SPELL == 3
+
 
 # ---------------------------------------------------------------------------
 # Grid analysis tests