S3 attachment storage

[binwiederhier]

Summary

Adds S3-compatible object storage as a backend for attachment caching, as an alternative to local filesystem storage.

• Minimal S3 client (s3/ package): implements PutObject (with automatic multipart upload for large files), GetObject, DeleteObjects, and ListObjectsV2 using AWS Signature V4 signing — no AWS SDK dependency
• Attachment store (attachment/ package): unified Store with pluggable backend interface (fileBackend / s3Backend), shared size tracking, rate limiting, and background sync
• Background cleanup: every 15 minutes, reconciles the S3 bucket (or prefix) with the message database — deletes orphaned objects whose file IDs are no longer referenced, and aborts incomplete multipart uploads older than 1 hour
• Configuration: attachment-cache-dir accepts an S3 URL (s3://ACCESS_KEY:SECRET_KEY@BUCKET[/PREFIX]?region=REGION[&endpoint=ENDPOINT]), auto-detecting path-style addressing when a custom endpoint is set
• Documentation: updated docs/config.md with S3 configuration examples and cleanup behavior

S3 client features

• AWS Signature V4 request signing (no external dependencies)
• Simple PUT for small objects, automatic multipart upload (5 MB parts) for large objects
• Batch delete (up to 1000 keys per call)
• Paginated ListObjectsV2 with prefix filtering
• ListMultipartUploads + AbortMultipartUpload for cleaning up incomplete uploads
• Path-style and virtual-hosted-style addressing
• Comprehensive test suite with mock S3 server

Test plan

• go test ./s3/... — S3 client unit tests (signing, URL construction, prefix handling) and integration tests (mock S3 server)
• go test ./attachment/... — attachment store tests for both file and S3 backends
• go test ./server/... — server tests pass
• go test ./util/... — LimitReader, CountingReader, and LimitWriter tests