feat(db): add storage backend metadata columns

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

migrations/017_file_storage_metadata.sql

@@ -0,0 +1,7 @@
+-- Track which storage backend holds each attached file.
+ALTER TABLE item_files
+  ADD COLUMN IF NOT EXISTS storage_backend TEXT NOT NULL DEFAULT 'minio';
+
+-- Track which storage backend holds each revision file.
+ALTER TABLE revisions
+  ADD COLUMN IF NOT EXISTS file_storage_backend TEXT NOT NULL DEFAULT 'minio';