Split mint mod.rs into separate trait modules

Split the large monolithic mod.rs file into separate files, each containing a
database trait and its corresponding transaction trait:

- proofs.rs: MintProofsTransaction + MintProofsDatabase
- keys.rs: MintKeyDatabaseTransaction + MintKeysDatabase
- quotes.rs: MintQuotesTransaction + MintQuotesDatabase
- signatures.rs: MintSignatureTransaction + MintSignaturesDatabase
- keyvalue.rs: KVStoreTransaction + KVStoreDatabase + KVStore
- saga.rs: SagaTransaction + SagaDatabase
- completed_operations.rs: CompletedOperationsTransaction +
  CompletedOperationsDatabase

The mod.rs now only contains struct definitions, migrations, and the main
MintDatabase/MintTransaction/DbTransactionFinalizer impls.

crates/cdk-sql-common/src/mint/mod.rs

@@ -13,7 +13,6 @@ use std::sync::Arc;
 
 use async_trait::async_trait;
 use cdk_common::database::{self, DbTransactionFinalizer, Error, MintDatabase};
-// Re-export for auth module
 use migrations::MIGRATIONS;
 
 use crate::common::migrate;