refactor db transactions (#571)

cashu/mint/protocols.py

@@ -4,6 +4,8 @@ from ..core.base import Method, MintKeyset, Unit
 from ..core.db import Database
 from ..lightning.base import LightningBackend
 from ..mint.crud import LedgerCrud
+from .db.read import DbReadHelper
+from .db.write import DbWriteHelper
 from .events.events import LedgerEventManager
 
 
@@ -18,6 +20,8 @@ class SupportsBackends(Protocol):
 
 class SupportsDb(Protocol):
     db: Database
+    db_read: DbReadHelper
+    db_write: DbWriteHelper
     crud: LedgerCrud