feat: harden download and storage resilience

[vsxd]

Summary

• add persisted download readiness state for skill versions and use it for list/download availability
• harden download delivery with lazy stream opening, bundle fallback metrics, and per-resource rate limit keys while preserving anonymous dual-bucket limiting
• translate storage failures into a unified 503 response and add S3 client timeout/connection tuning

Testing

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