feat(storage): support AWS S3 NoAk default credentials

[qilifan]

Description

Support AWS S3 authentication without persisted access keys by preserving the AWS SDK for Go v2 default credential chain when Access Key and Secret Key are both empty. This enables EC2/ECS IAM roles, EKS IRSA/Web Identity, environment credentials, and shared AWS configuration while keeping existing static AK/SK configurations backward compatible.

This change also makes the standard AWS S3 endpoint optional, updates tenant/global validation and connectivity checks, adjusts the storage settings UI and translations, and documents the environment-variable behavior.

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 💥 Breaking change
• 📚 Documentation update
• 🎨 Refactor
• ⚡ Performance improvement
• 🧪 Test
• 🔧 Configuration / Build / CI

Related Issue

N/A

Testing

• gofmt completed with no diff
• git diff --check passed
• bash -n scripts/check-env.sh passed
• go test ./internal/application/service/file passed
• go test ./internal/handler ./internal/container -run ^$ passed
• npm ci completed; 0 vulnerabilities
• npm run type-check runs but the repository currently has two unrelated pre-existing errors in KnowledgeBaseList.vue at lines 439 and 592

Checklist

• Self-reviewed the code
• Added/updated tests covering the change
• Updated related documentation and UI translations
• No breaking change introduced
• make fmt && make lint && make test pass locally (targeted affected-package checks pass; full frontend type-check is blocked by two unrelated pre-existing errors noted above)

Screenshots / Recordings

Not included. The UI change only marks S3 credentials and endpoint as optional and adds explanatory help text.

[lyingbug]

处理下静态检查，另外i18n里并不需要引入如此多的diff，只增加需要的

[qilifan]

@lyingbug 处理好了