feat: added db transactions abstraction; cleaned up user registration; repos adapted new abstraction: all repos now support transactions, if present in ctx

internal/domain/transactor.go

@@ -0,0 +1,7 @@
+package domain
+
+import "context"
+
+type Transactor interface {
+	WithinTx(ctx context.Context, fn func(ctx context.Context) error) error
+}