feat(oss): add native Alibaba Cloud OSS storage backend

[corylanou]

Summary

• Add native support for Alibaba Cloud Object Storage Service (OSS) as a replication backend
• Uses official alibabacloud-oss-go-sdk-v2 SDK instead of S3-compatible mode
• Includes multipart upload support for large files (>5GB) with configurable part-size/concurrency
• Preserves timestamps via OSS metadata for accurate point-in-time recovery

Background

Litestream 0.5+ uses AWS SDK Go v2, which enables chunked encoding for uploads ≥5MB. Alibaba Cloud OSS doesn't support chunked encoding via its S3-compatible mode, breaking OSS users who upgraded from 0.3.x.

Changes

New Files

• oss/replica_client.go - Main OSS ReplicaClient implementation
• oss/replica_client_test.go - Unit tests

Modified Files

• cmd/litestream/main.go - Add OSS config parsing and registration
• cmd/litestream/replicate.go - Add OSS logging
• internal/testingutil/testingutil.go - Add OSS integration test support
• go.mod, go.sum - Add Alibaba SDK dependency
• CLAUDE.md, AGENTS.md, docs/ARCHITECTURE.md - Documentation updates

Configuration

# Explicit configuration
dbs:
  - path: /data/mydb.sqlite
    replicas:
      - type: oss
        bucket: my-bucket
        region: cn-hangzhou
        part-size: 10485760   # Optional: 10MB parts
        concurrency: 5        # Optional: parallel uploads

# URL format
dbs:
  - path: /data/mydb.sqlite
    replicas:
      - url: oss://my-bucket.oss-cn-hangzhou.aliyuncs.com/backups

Environment Variables

Variable	Description
LITESTREAM_OSS_ACCESS_KEY_ID	Alibaba Cloud access key ID
LITESTREAM_OSS_ACCESS_KEY_SECRET	Alibaba Cloud access key secret
LITESTREAM_OSS_REGION	OSS region (e.g., cn-hangzhou)
LITESTREAM_OSS_BUCKET	OSS bucket name

Test plan

• Unit tests pass (go test -v ./oss/...)
• Build passes (go build ./...)
• Linters pass (go vet, staticcheck)
• Pre-commit hooks pass
• Integration tests with real OSS bucket (requires credentials)

Closes #861

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