feat: Add SDK Protection Middleware against systematic enumeration

Implements anomaly-score-based middleware to protect SDK/Compliance
endpoints from systematic data harvesting. Includes 5 detection
mechanisms (diversity, burst, sequential enumeration, unusual hours,
multi-tenant), multi-window quota system, progressive throttling,
HMAC watermarking, and graceful Valkey fallback.

- backend/middleware/sdk_protection.py: Core middleware (~750 lines)
- Admin API endpoints for score management and tier configuration
- 14 new tests (all passing)
- MkDocs documentation with clear explanations
- Screen flow and middleware dashboard updates

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/main.py

@@ -3,6 +3,8 @@ from fastapi.middleware.cors import CORSMiddleware
 from fastapi.staticfiles import StaticFiles
 from pathlib import Path
 
+from middleware import SDKProtectionMiddleware
+
 from original_service import router as original_router
 from learning_units_api import router as learning_units_router
 from frontend.studio import router as studio_router
@@ -60,6 +62,9 @@ app.add_middleware(
     allow_headers=["*"],
 )
 
+# SDK Protection Middleware (Schutz vor systematischer Enumeration)
+app.add_middleware(SDKProtectionMiddleware)
+
 # Hier hängen wir die einzelnen Service-Router ein.
 # Alle Routen bekommen das Präfix /api, damit das Frontend sie findet.
 app.include_router(original_router, prefix="/api")