Add storage_backend columns to track which backend (minio or filesystem)
holds each file, enabling dual-running during migration.
Migration 017_file_storage_metadata.sql:
- item_files.storage_backend TEXT NOT NULL DEFAULT 'minio'
- revisions.file_storage_backend TEXT NOT NULL DEFAULT 'minio'
DB repository changes:
- Revision struct: add FileStorageBackend field
- ItemFile struct: add StorageBackend field
- All INSERT queries include the new columns
- All SELECT queries read them (COALESCE for pre-migration compat)
- CreateRevisionFromExisting copies the backend from source revision
- Default to 'minio' when field is empty (backward compat)
Existing rows default to 'minio'. New uploads will write 'filesystem'
when the filesystem backend is active.
Closes#128
forbes-0023
8cef4fa55f