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/lib/navigation.ts

@@ -150,9 +150,18 @@ export const navigation: NavCategory[] = [
         audience: ['Entwickler', 'Data Scientists'],
         subgroup: 'KI-Werkzeuge',
       },
+      {
+        id: 'ocr-kombi',
+        name: 'OCR Kombi',
+        href: '/ai/ocr-kombi',
+        description: 'Modulare 11-Schritt-Pipeline',
+        purpose: 'Modulare OCR-Pipeline mit Dual-Engine (PP-OCRv5 + Tesseract), Strukturerkennung, Grid-Aufbau und Review. Multi-Page-Dokument-Unterstuetzung.',
+        audience: ['Entwickler'],
+        subgroup: 'KI-Werkzeuge',
+      },
       {
         id: 'ocr-overlay',
-        name: 'OCR Overlay',
+        name: 'OCR Overlay (Legacy)',
         href: '/ai/ocr-overlay',
         description: 'Ganzseitige Overlay-Rekonstruktion',
         purpose: 'Arbeitsblatt ohne Spaltenerkennung direkt als Overlay rekonstruieren. Vereinfachte 7-Schritt-Pipeline.',