Fix: preserve = ; : - and other meaningful symbols in word_boxes

Rule (a2) in Step 5i removed word_boxes with no letters/digits as
"graphic OCR artifacts". This incorrectly removed = signs used as
definition markers in textbooks ("film = 1. Film; 2. filmen").

Added exception list _KEEP_SYMBOLS for meaningful punctuation:
= (= =) ; : - – — / + • · ( ) & * → ← ↔

The root cause: PaddleOCR returns "film = 1. Film; 2. filmen" as one
block, which gets split into word_boxes ["film", "=", "1.", ...].
The "=" word_box had no alphanumeric chars and was removed as artifact.

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

klausur-service/backend/grid_build_core.py

@@ -1407,9 +1407,13 @@ async def _build_grid_core(
             # Rule (a2): isolated non-alphanumeric symbols (graphic OCR artifacts)
             # Small images/icons next to words get OCR'd as ">", "<", "~", etc.
             # Remove word boxes that contain NO letters or digits.
+            # Exception: meaningful punctuation used in textbooks (=, ;, :, -, etc.)
+            _KEEP_SYMBOLS = {'=', '(=', '=)', ';', ':', '-', '–', '—', '/', '+',
+                             '•', '·', '(', ')', '&', '*', '→', '←', '↔'}
             for i, wb in enumerate(wbs):
                 t = (wb.get("text") or "").strip()
                 if t and not re.search(r'[a-zA-Z0-9äöüÄÖÜß]', t) and len(t) <= 2:
+                    if t not in _KEEP_SYMBOLS:
                         to_remove.add(i)
 
             # Rule (b) + (c): overlap and duplicate detection