Add OCR Kombi Pipeline: modular 11-step architecture with multi-page support

Phase 1 of the clean architecture refactor: Replaces the 751-line ocr-overlay
monolith with a modular pipeline. Each step gets its own component file.

Frontend: /ai/ocr-kombi route with 11 steps (Upload, Orientation, PageSplit,
Deskew, Dewarp, ContentCrop, OCR, Structure, GridBuild, GridReview, GroundTruth).
Session list supports document grouping for multi-page uploads.

Backend: New ocr_kombi/ module with multi-page PDF upload (splits PDF into N
sessions with shared document_group_id). DB migration adds document_group_id
and page_number columns.

Old /ai/ocr-overlay remains fully functional for A/B testing.

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

klausur-service/backend/main.py

@@ -46,6 +46,7 @@ from ocr_pipeline_api import router as ocr_pipeline_router, _cache as ocr_pipeli
 from grid_editor_api import router as grid_editor_router
 from orientation_crop_api import router as orientation_crop_router, set_cache_ref as set_orientation_crop_cache
 from ocr_pipeline_session_store import init_ocr_pipeline_tables
+from ocr_kombi.router import router as ocr_kombi_router
 try:
     from handwriting_htr_api import router as htr_router
 except ImportError:
@@ -186,6 +187,7 @@ if htr_router:
     app.include_router(htr_router)            # Handwriting HTR (Klausur)
 if dsfa_rag_router:
     app.include_router(dsfa_rag_router)   # DSFA RAG Corpus Search
+app.include_router(ocr_kombi_router)      # OCR Kombi Pipeline (modular)
 
 
 # =============================================