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/ocr_kombi/router.py

@@ -0,0 +1,19 @@
+"""
+Composite router for the OCR Kombi pipeline.
+
+Aggregates step-specific sub-routers into one router for main.py to include.
+"""
+
+from fastapi import APIRouter
+
+from .step_upload import router as upload_router
+
+router = APIRouter(prefix="/api/v1/ocr-kombi", tags=["ocr-kombi"])
+
+# Include step-specific routes
+router.include_router(upload_router)
+
+# Future steps will be added here:
+# from .step_orientation import router as orientation_router
+# router.include_router(orientation_router)
+# ...