feat(storage): support S3 IAM role credentials

[nitin-bhadauria]

Summary

• add BACKEND_STORAGE_S3_USE_IAM_ROLE to enable the AWS SDK default credential provider chain for S3 storage
• omit explicit S3 access key/secret key credentials when IAM role mode is enabled
• document the env flag and cover static/IAM S3 client construction with unit tests

Tests

• pnpm -F @teable/backend test-unit src/features/attachments/plugins/s3.spec.ts
• pnpm -C apps/nestjs-backend exec eslint src/features/attachments/plugins/s3.ts src/features/attachments/plugins/s3.spec.ts --ext .ts,.js,.cjs,.mjs,.mdx

Note: pnpm -F @teable/backend typecheck still fails on existing upstream baseline spec typing errors unrelated to this change.

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