feat(ocr-pipeline): British/American IPA pronunciation choice

- Integrate Britfone dictionary (MIT, 15k British English IPA entries)
- Add pronunciation parameter: 'british' (default) or 'american'
- British uses Britfone (Received Pronunciation), falls back to CMU
- American uses eng_to_ipa/CMU, falls back to Britfone
- Frontend: dropdown to switch pronunciation, default = British
- API: ?pronunciation=british|american query parameter

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

klausur-service/backend/ocr_pipeline_api.py

@@ -1007,11 +1007,12 @@ async def get_row_ground_truth(session_id: str):
 # ---------------------------------------------------------------------------
 
 @router.post("/sessions/{session_id}/words")
-async def detect_words(session_id: str, engine: str = "auto"):
+async def detect_words(session_id: str, engine: str = "auto", pronunciation: str = "british"):
     """Build word grid from columns × rows, OCR each cell.
 
     Query params:
         engine: 'auto' (default), 'tesseract', or 'rapid'
+        pronunciation: 'british' (default) or 'american' — for IPA dictionary lookup
     """
     if session_id not in _cache:
         await _load_session_to_cache(session_id)
@@ -1068,6 +1069,7 @@ async def detect_words(session_id: str, engine: str = "auto"):
     entries = build_word_grid(
         ocr_img, col_regions, row_geoms, img_w, img_h,
         ocr_engine=engine, img_bgr=dewarped_bgr,
+        pronunciation=pronunciation,
     )
     duration = time.time() - t0