refactor: remove unused pages and backends (model-management, OCR legacy, GPU/vast.ai, video-chat, matrix)

Deleted pages:
- /ai/model-management (mock data only, no real backend)
- /ai/ocr-compare (old /vocab/ backend, replaced by ocr-kombi)
- /ai/ocr-pipeline (minimal session browser, redundant)
- /ai/ocr-overlay (legacy monolith, redundant)
- /ai/gpu (vast.ai GPU management, no longer used)
- /infrastructure/gpu (same)
- /communication/video-chat (moved to core)
- /communication/matrix (moved to core)

Deleted backends:
- backend-lehrer/infra/vast_client.py + vast_power.py
- backend-lehrer/meetings_api.py + jitsi_api.py
- website/app/api/admin/gpu/
- edu-search-service/scripts/vast_ai_extractor.py

Total: ~7,800 LOC removed. All code preserved in git history.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

admin-lehrer/app/(admin)/ai/ocr-kombi/page.tsx

@@ -127,7 +127,6 @@ function OcrKombiContent() {
           databases: ['PostgreSQL Sessions'],
         }}
         relatedPages={[
-          { name: 'OCR Overlay (Legacy)', href: '/ai/ocr-overlay', description: 'Alter 3-Modi-Monolith' },
           { name: 'OCR Regression', href: '/ai/ocr-regression', description: 'Regressionstests' },
         ]}
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