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>

backend-lehrer/main.py

@@ -40,7 +40,6 @@ os.environ["DATABASE_URL"] = DATABASE_URL
 # ---------------------------------------------------------------------------
 LLM_GATEWAY_ENABLED = os.getenv("LLM_GATEWAY_ENABLED", "false").lower() == "true"
 ALERTS_AGENT_ENABLED = os.getenv("ALERTS_AGENT_ENABLED", "false").lower() == "true"
-VAST_API_KEY = os.getenv("VAST_API_KEY")
 
 
 # ---------------------------------------------------------------------------
@@ -116,15 +115,10 @@ app.include_router(unit_router)  # Already has /api/units prefix
 from unit_analytics_api import router as unit_analytics_router
 app.include_router(unit_analytics_router)  # Already has /api/analytics prefix
 
-# --- 5. Meetings / Jitsi ---
-from meetings_api import router as meetings_api_router
-app.include_router(meetings_api_router)  # Already has /api/meetings prefix
 
 from recording_api import router as recording_api_router
 app.include_router(recording_api_router)  # Already has /api/recordings prefix
 
-from jitsi_api import router as jitsi_router
-app.include_router(jitsi_router)  # Already has /api/jitsi prefix
 
 # --- 6. Messenger ---
 from messenger_api import router as messenger_router
@@ -184,11 +178,6 @@ if ALERTS_AGENT_ENABLED:
     from alerts_agent.api import router as alerts_router
     app.include_router(alerts_router, prefix="/api", tags=["Alerts Agent"])
 
-# --- 14. vast.ai GPU Infrastructure (optional) ---
-if VAST_API_KEY:
-    from infra.vast_power import router as vast_router
-    app.include_router(vast_router, tags=["GPU Infrastructure"])
-
 
 # ---------------------------------------------------------------------------
 # Middleware (from shared middleware/ package)