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/StepOcr.tsx

@@ -0,0 +1,30 @@
+'use client'
+
+import { PaddleDirectStep } from '@/components/ocr-overlay/PaddleDirectStep'
+
+interface StepOcrProps {
+  sessionId: string | null
+  onNext: () => void
+}
+
+/**
+ * Step 7: OCR (Kombi mode = PaddleOCR + Tesseract).
+ *
+ * Phase 1: Uses the existing PaddleDirectStep with kombi endpoint.
+ * Phase 3 (later) will add transparent 3-phase progress + engine comparison.
+ */
+export function StepOcr({ sessionId, onNext }: StepOcrProps) {
+  return (
+    <PaddleDirectStep
+      sessionId={sessionId}
+      onNext={onNext}
+      endpoint="paddle-kombi"
+      title="Kombi-Modus"
+      description="PP-OCRv5 und Tesseract laufen parallel. Koordinaten werden gewichtet gemittelt fuer optimale Positionierung."
+      icon="🔀"
+      buttonLabel="PP-OCRv5 + Tesseract starten"
+      runningLabel="PP-OCRv5 + Tesseract laufen..."
+      engineKey="kombi"
+    />
+  )
+}