Feature: implement S3-compatible backend (AWS S3, Backblaze B2, Cloudflare R2)

[piranhap]

Summary

src/backend/s3.rs is currently a stub returning NotImplemented. An S3-compatible backend would cover AWS S3, Backblaze B2, Cloudflare R2, and MinIO — making opendotsync viable without rclone or GitHub for users who already have cloud storage.

Config

backend:
  type: s3
  bucket: my-dotsync-bucket
  region: us-east-1           # optional, defaults to us-east-1
  prefix: opendotsync/        # optional path prefix within bucket
  endpoint: https://...       # optional, for R2/B2/MinIO custom endpoints

Credentials via standard AWS env vars: AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY, or ~/.aws/credentials.

Implementation Notes

• Add aws-sdk-s3 behind the existing backend-s3 feature flag in Cargo.toml
• Add prefix and endpoint fields to BackendConfig::S3 in src/config.rs
• Object layout mirrors the local backend: <prefix>/<profile>/manifest.json, <prefix>/<profile>/v001.tar.zst
• push: PutObject with the snapshot bytes
• pull: GetObject
• prune: DeleteObject for each old version
• load_manifest / save_manifest: GetObject / PutObject on manifest.json; return default manifest on NoSuchKey
• Use aws_sdk_s3::Client::from_env() for zero-config credential resolution
• Wire in create_backend() and cmd_init() in src/sync.rs

Affected Files

• Cargo.toml — add aws-sdk-s3, aws-config, tokio behind backend-s3 feature
• src/config.rs — add prefix, endpoint fields to BackendConfig::S3
• src/backend/s3.rs — full implementation
• src/sync.rs — wire S3Backend in create_backend() and cmd_init()

Note on Async

aws-sdk-s3 is async. StorageBackend is currently sync. Options:

1. Add a tokio runtime and block inside the S3 backend methods (tokio::runtime::Handle::current().block_on(...))
2. Make StorageBackend async (larger refactor, better long-term)

Option 1 is recommended for now to avoid touching the trait and all existing backends.