feat(rate-limiting): Implement core rate limiting functionality with configuration, decision-making, metrics, middleware, and service registration

- Add RateLimitConfig for configuration management with YAML binding support.
- Introduce RateLimitDecision to encapsulate the result of rate limit checks.
- Implement RateLimitMetrics for OpenTelemetry metrics tracking.
- Create RateLimitMiddleware for enforcing rate limits on incoming requests.
- Develop RateLimitService to orchestrate instance and environment rate limit checks.
- Add RateLimitServiceCollectionExtensions for dependency injection registration.

docs/modules/vexlens/architecture.md

@@ -43,7 +43,7 @@
 * **Vuln Explorer**: Enriches vulnerability data with VEX status.
 * **Orchestrator**: Schedules consensus compute jobs for batch processing.
 * **Authority**: Validates issuer trust and key fingerprints.
-* **Config stores**: MongoDB (projections, issuer directory), Redis (caches).
+* **Config stores**: PostgreSQL (projections, issuer directory), Redis (caches).
 
 ---
 
@@ -168,7 +168,7 @@ vexlens:
     projectionRetentionDays: 365
     eventRetentionDays: 90
   issuerDirectory:
-    source: mongodb  # mongodb, file, api
+    source: postgresql  # postgresql, file, api
     refreshIntervalMinutes: 60
 ```