Add multi-database data layer

Introduce aether-data-schema and driver-specific schema generation for Postgres, MySQL, and SQLite.

Split data backends, lifecycle, repositories, and gateway runtime integration across database drivers.

Verified with cargo fmt --all --check, cargo clippy --workspace --all-targets -- -D warnings, and cargo test --workspace.

crates/aether-data/src/repository/wallet/mod.rs

@@ -1,9 +1,13 @@
 mod memory;
-mod sql;
+mod mysql;
+mod postgres;
+mod sqlite;
 mod types;
 
 pub use memory::InMemoryWalletRepository;
-pub use sql::SqlxWalletRepository;
+pub use mysql::MysqlWalletReadRepository;
+pub use postgres::SqlxWalletRepository;
+pub use sqlite::SqliteWalletReadRepository;
 pub use types::{
     AdjustWalletBalanceInput, AdminPaymentCallbackRecord, AdminPaymentOrderListQuery,
     AdminRedeemCodeBatchListQuery, AdminRedeemCodeListQuery, AdminWalletLedgerQuery,