Add pipe auto-correction and graphic artifact filter for grid builder

- autocorrect_pipe_artifacts(): strips OCR pipe artifacts from printed
  syllable dividers, validates with pyphen, tries char-deletion near
  pipe positions for garbled words (e.g. "Ze|plpe|lin" → "Zeppelin")
- Rule (a2): filters isolated non-alphanumeric word boxes (≤2 chars,
  no letters/digits) — catches small icons OCR'd as ">", "<" etc.
- Both fixes are generic: pyphen-validated, no session-specific logic

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

klausur-service/backend/grid_editor_api.py

@@ -1323,6 +1323,14 @@ async def _build_grid_core(
                         and wb.get("conf", 100) < 85):
                     to_remove.add(i)
 
+            # 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.
+            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:
+                    to_remove.add(i)
+
             # Rule (b) + (c): overlap and duplicate detection
             # Sort by x for pairwise comparison
             _ALPHA_WORD_RE = re.compile(r'^[A-Za-z\u00c0-\u024f\-]+[.,;:!?]*$')
@@ -1619,6 +1627,15 @@ async def _build_grid_core(
     except Exception as e:
         logger.warning("Word-gap merge failed: %s", e)
 
+    # --- Pipe auto-correction: fix OCR artifacts from printed syllable dividers ---
+    # Strips | from words, validates with pyphen, tries char-deletion for garbled
+    # words like "Ze|plpe|lin" → "Zeppelin".
+    try:
+        from cv_syllable_detect import autocorrect_pipe_artifacts
+        autocorrect_pipe_artifacts(zones_data, session_id)
+    except Exception as e:
+        logger.warning("Pipe autocorrect failed: %s", e)
+
     # --- Syllable divider insertion for dictionary pages ---
     # syllable_mode: "auto" = only when original has pipe dividers (1% threshold),
     #   "all" = force on all content words, "en" = English column only,