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>

admin-lehrer/components/ocr-kombi/StepStructure.tsx

@@ -0,0 +1,13 @@
+'use client'
+
+import { StepStructureDetection } from '@/components/ocr-pipeline/StepStructureDetection'
+
+interface StepStructureProps {
+  sessionId: string | null
+  onNext: () => void
+}
+
+/** Thin wrapper around the shared StepStructureDetection component */
+export function StepStructure({ sessionId, onNext }: StepStructureProps) {
+  return <StepStructureDetection sessionId={sessionId} onNext={onNext} />
+}