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/middleware/__init__.py

@@ -7,6 +7,7 @@ This module provides middleware components for the FastAPI backend:
 - Rate Limiter: Protects against abuse (Valkey-based)
 - PII Redactor: Redacts sensitive data from logs
 - Input Gate: Validates request body size and content types
+- SDK Protection: Protects SDK endpoints from systematic enumeration
 """
 
 from .request_id import RequestIDMiddleware, get_request_id
@@ -14,6 +15,7 @@ from .security_headers import SecurityHeadersMiddleware
 from .rate_limiter import RateLimiterMiddleware
 from .pii_redactor import PIIRedactor, redact_pii
 from .input_gate import InputGateMiddleware
+from .sdk_protection import SDKProtectionMiddleware, SDKProtectionConfig
 
 __all__ = [
     "RequestIDMiddleware",
@@ -23,4 +25,6 @@ __all__ = [
     "PIIRedactor",
     "redact_pii",
     "InputGateMiddleware",
+    "SDKProtectionMiddleware",
+    "SDKProtectionConfig",
 ]