feat: harden download and storage resilience

[vsxd]

Summary

• harden S3/object-storage access with explicit connection pool and timeout settings
• persist skill version download readiness, remove version-list S3 probing, and keep bundle-missing fallback with warnings
• add download/storage/rate-limit metrics, map storage failures to 503, tighten download rate limiting, and document the new operational knobs

Testing

• ./mvnw -pl skillhub-storage,skillhub-domain -am -Dtest=SkillQueryServiceTest,SkillDownloadServiceTest -Dsurefire.failIfNoSpecifiedTests=false test
• ./mvnw -pl skillhub-app -am -Dtest=SkillControllerDownloadTest,SkillControllerTest,AuthRateLimitControllerTest -Dsurefire.failIfNoSpecifiedTests=false test