Introduces a StorageBackend interface with local filesystem and S3
implementations. The StorageService delegates raw I/O to the backend
while keeping validation, encryption, and URL generation unchanged.
Backend selection is config-driven: set B2_ENDPOINT + B2_KEY_ID +
B2_APP_KEY + B2_BUCKET_NAME for S3 mode, or STORAGE_UPLOAD_DIR for
local mode. STORAGE_USE_SSL=false for in-cluster MinIO (HTTP).
All existing tests pass unchanged — the local backend preserves
identical behavior to the previous direct-filesystem implementation.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
