Add CORS support and response caching to aggregate endpoints

- Introduced CORS configuration options in config.example.yaml, allowing specification of allowed origins for cross-origin requests.
- Enhanced the aggregate handler to support response caching with a configurable TTL, improving performance for repeated requests.
- Updated the aggregate API to return a structured response indicating whether any upstream requests failed, enhancing error handling and response clarity.
- Modified documentation in AGGREGATE.md and README.md to reflect the new CORS and caching features.
- Added tests to validate the new functionality in the aggregate handler.

config.example.yaml

@@ -23,11 +23,17 @@ whitelist_cidrs:
 #   - "10.0.0.0/8"
 trusted_proxies: []
 
-# Опционально: по умолчанию /api/agg/* опрашивает все servers; можно ограничить список:
+# SPA на другом origin (preflight OPTIONS + Access-Control-Allow-Origin):
+# cors_allowed_origins:
+#   - "http://localhost:5173"
+#   # - "*"
+
+# Опционально: по умолчанию /api/agg/* опрашивает все servers; можно ограничить список и включить кэш ответов:
 # aggregate:
 #   include_aliases:
 #     - gt1
 #     - gt2
+#   cache_ttl_ms: 2000
 
 # Геолокация IP в /api/agg/unique-ips. См. docs/GEOIP.md (City + опционально ASN; Country-only не нужен)
 # geoip: