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

internal/db/items.go

@@ -35,11 +35,12 @@ type Revision struct {
 	ItemID         string
 	RevisionNumber int
 	Properties     map[string]any
-	FileKey        *string
-	FileVersion    *string
-	FileChecksum   *string
-	FileSize       *int64
-	ThumbnailKey   *string
+	FileKey            *string
+	FileVersion        *string
+	FileChecksum       *string
+	FileSize           *int64
+	FileStorageBackend string // "minio" or "filesystem"
+	ThumbnailKey       *string
 	CreatedAt      time.Time
 	CreatedBy      *string
 	Comment        *string
@@ -306,16 +307,20 @@ func (r *ItemRepository) CreateRevision(ctx context.Context, rev *Revision) erro
 		return fmt.Errorf("marshaling properties: %w", err)
 	}
 
+	if rev.FileStorageBackend == "" {
+		rev.FileStorageBackend = "minio"
+	}
+
 	err = r.db.pool.QueryRow(ctx, `
 		INSERT INTO revisions (
 			item_id, revision_number, properties, file_key, file_version,
-			file_checksum, file_size, thumbnail_key, created_by, comment
+			file_checksum, file_size, file_storage_backend, thumbnail_key, created_by, comment
 		)
-		SELECT $1, current_revision + 1, $2, $3, $4, $5, $6, $7, $8, $9
+		SELECT $1, current_revision + 1, $2, $3, $4, $5, $6, $7, $8, $9, $10
 		FROM items WHERE id = $1
 		RETURNING id, revision_number, created_at
 	`, rev.ItemID, propsJSON, rev.FileKey, rev.FileVersion,
-		rev.FileChecksum, rev.FileSize, rev.ThumbnailKey, rev.CreatedBy, rev.Comment,
+		rev.FileChecksum, rev.FileSize, rev.FileStorageBackend, rev.ThumbnailKey, rev.CreatedBy, rev.Comment,
 	).Scan(&rev.ID, &rev.RevisionNumber, &rev.CreatedAt)
 	if err != nil {
 		return fmt.Errorf("inserting revision: %w", err)
@@ -342,7 +347,8 @@ func (r *ItemRepository) GetRevisions(ctx context.Context, itemID string) ([]*Re
 	if hasStatusColumn {
 		rows, err = r.db.pool.Query(ctx, `
 			SELECT id, item_id, revision_number, properties, file_key, file_version,
-			       file_checksum, file_size, thumbnail_key, created_at, created_by, comment,
+			       file_checksum, file_size, COALESCE(file_storage_backend, 'minio'),
+			       thumbnail_key, created_at, created_by, comment,
 			       COALESCE(status, 'draft') as status, COALESCE(labels, ARRAY[]::TEXT[]) as labels
 			FROM revisions
 			WHERE item_id = $1
@@ -369,7 +375,8 @@ func (r *ItemRepository) GetRevisions(ctx context.Context, itemID string) ([]*Re
 		if hasStatusColumn {
 			err = rows.Scan(
 				&rev.ID, &rev.ItemID, &rev.RevisionNumber, &propsJSON, &rev.FileKey, &rev.FileVersion,
-				&rev.FileChecksum, &rev.FileSize, &rev.ThumbnailKey, &rev.CreatedAt, &rev.CreatedBy, &rev.Comment,
+				&rev.FileChecksum, &rev.FileSize, &rev.FileStorageBackend,
+				&rev.ThumbnailKey, &rev.CreatedAt, &rev.CreatedBy, &rev.Comment,
 				&rev.Status, &rev.Labels,
 			)
 		} else {
@@ -379,6 +386,7 @@ func (r *ItemRepository) GetRevisions(ctx context.Context, itemID string) ([]*Re
 			)
 			rev.Status = "draft"
 			rev.Labels = []string{}
+			rev.FileStorageBackend = "minio"
 		}
 		if err != nil {
 			return nil, fmt.Errorf("scanning revision: %w", err)
@@ -412,13 +420,15 @@ func (r *ItemRepository) GetRevision(ctx context.Context, itemID string, revisio
 	if hasStatusColumn {
 		err = r.db.pool.QueryRow(ctx, `
 			SELECT id, item_id, revision_number, properties, file_key, file_version,
-			       file_checksum, file_size, thumbnail_key, created_at, created_by, comment,
+			       file_checksum, file_size, COALESCE(file_storage_backend, 'minio'),
+			       thumbnail_key, created_at, created_by, comment,
 			       COALESCE(status, 'draft') as status, COALESCE(labels, ARRAY[]::TEXT[]) as labels
 			FROM revisions
 			WHERE item_id = $1 AND revision_number = $2
 		`, itemID, revisionNumber).Scan(
 			&rev.ID, &rev.ItemID, &rev.RevisionNumber, &propsJSON, &rev.FileKey, &rev.FileVersion,
-			&rev.FileChecksum, &rev.FileSize, &rev.ThumbnailKey, &rev.CreatedAt, &rev.CreatedBy, &rev.Comment,
+			&rev.FileChecksum, &rev.FileSize, &rev.FileStorageBackend,
+			&rev.ThumbnailKey, &rev.CreatedAt, &rev.CreatedBy, &rev.Comment,
 			&rev.Status, &rev.Labels,
 		)
 	} else {
@@ -433,6 +443,7 @@ func (r *ItemRepository) GetRevision(ctx context.Context, itemID string, revisio
 		)
 		rev.Status = "draft"
 		rev.Labels = []string{}
+		rev.FileStorageBackend = "minio"
 	}
 
 	if err == pgx.ErrNoRows {
@@ -606,15 +617,16 @@ func (r *ItemRepository) CreateRevisionFromExisting(ctx context.Context, itemID
 
 	// Create new revision with copied properties (and optionally file reference)
 	newRev := &Revision{
-		ItemID:       itemID,
-		Properties:   source.Properties,
-		FileKey:      source.FileKey,
-		FileVersion:  source.FileVersion,
-		FileChecksum: source.FileChecksum,
-		FileSize:     source.FileSize,
-		ThumbnailKey: source.ThumbnailKey,
-		CreatedBy:    createdBy,
-		Comment:      &comment,
+		ItemID:             itemID,
+		Properties:         source.Properties,
+		FileKey:            source.FileKey,
+		FileVersion:        source.FileVersion,
+		FileChecksum:       source.FileChecksum,
+		FileSize:           source.FileSize,
+		FileStorageBackend: source.FileStorageBackend,
+		ThumbnailKey:       source.ThumbnailKey,
+		CreatedBy:          createdBy,
+		Comment:            &comment,
 	}
 
 	// Insert the new revision
@@ -626,13 +638,13 @@ func (r *ItemRepository) CreateRevisionFromExisting(ctx context.Context, itemID
 	err = r.db.pool.QueryRow(ctx, `
 		INSERT INTO revisions (
 			item_id, revision_number, properties, file_key, file_version,
-			file_checksum, file_size, thumbnail_key, created_by, comment, status
+			file_checksum, file_size, file_storage_backend, thumbnail_key, created_by, comment, status
 		)
-		SELECT $1, current_revision + 1, $2, $3, $4, $5, $6, $7, $8, $9, 'draft'
+		SELECT $1, current_revision + 1, $2, $3, $4, $5, $6, $7, $8, $9, $10, 'draft'
 		FROM items WHERE id = $1
 		RETURNING id, revision_number, created_at
 	`, newRev.ItemID, propsJSON, newRev.FileKey, newRev.FileVersion,
-		newRev.FileChecksum, newRev.FileSize, newRev.ThumbnailKey, newRev.CreatedBy, newRev.Comment,
+		newRev.FileChecksum, newRev.FileSize, newRev.FileStorageBackend, newRev.ThumbnailKey, newRev.CreatedBy, newRev.Comment,
 	).Scan(&newRev.ID, &newRev.RevisionNumber, &newRev.CreatedAt)
 	if err != nil {
 		return nil, fmt.Errorf("inserting revision: %w", err)