feat(backend): rouage gin /api/v1, enveloppe {data,meta}/{error}, health réel, stubs 501 + test de contrat routes

backend/ocr/engine.go

@@ -7,4 +7,4 @@ import "context"
 // swapping to a remote service later only changes the injection point.
 type Engine interface {
 	ExtractText(ctx context.Context, filePath string, lang string) (string, error)
-}
+}