docs: add reference Python/Flask implementation

Previous version of Tanabata used as visual and functional reference
for the new Go + SvelteKit rewrite.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/reference/backend/internal/storage/storage.go

@@ -0,0 +1,21 @@
+package storage
+
+import (
+	"encoding/json"
+	"time"
+
+	"tanabata/internal/domain"
+)
+
+type Storage interface {
+	FileRepository
+	Close()
+}
+
+type FileRepository interface {
+	GetSlice(user_id int, filter, sort string, limit, offset int) (files domain.Slice[domain.FileItem], statusCode int, err error)
+	Get(user_id int, file_id string) (file domain.FileFull, statusCode int, err error)
+	Add(user_id int, name, mime string, datetime time.Time, notes string, metadata json.RawMessage) (file domain.FileCore, statusCode int, err error)
+	Update(user_id int, file_id string, updates map[string]interface{}) (statusCode int, err error)
+	Delete(user_id int, file_id string) (statusCode int, err error)
+}