feat(storage): add Aliyun OSS backend with OSS_PRESIGN_DISABLED support

[ruanjf]

• Add server/internal/storage/oss.go: full OSS storage implementation using the Aliyun alibabacloud-oss-go-sdk-v2 SDK
• OSS_PRESIGN_DISABLED env var: when set to "true" or "1", PresignGetURL returns empty string instead of generating a signed URL
• Support AK+SK and ECS RAM role auth, CDN URL Auth (Type A), custom endpoints
• Wire OSSStorage into router as secondary option (S3 > OSS > local)
• Add OSS env vars to docker-compose.selfhost.yml including OSS_PRESIGN_DISABLED

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• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Refactor / code improvement (no behavior change)
• Documentation update
• Tests (adding or improving test coverage)
• CI / infrastructure

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