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/lib.rs

@@ -1,16 +1,31 @@
-pub mod backends;
-pub mod backfill;
+//! Runtime data access for Aether.
+//!
+//! This crate contains concrete database/Redis clients, repository
+//! implementations, migration/backfill/export workflows, and the backend
+//! composition layer. Shared repository contracts that other crates compile
+//! against live in `aether-data-contracts`.
+//!
+//! See `crates/aether-data/README.md` for the layer map and SQL driver policy.
+
+pub mod backend;
 mod config;
+mod database;
+pub mod driver;
 mod error;
-pub mod migrate;
-pub mod postgres;
-pub mod redis;
+pub mod lifecycle;
+pub mod maintenance;
 pub mod repository;
 
-pub use backends::{
+pub use backend::{
     DataBackends, DataLeaseBackends, DataLockBackends, DataReadRepositories,
     DataTransactionBackends, DataWorkerBackends, DataWriteRepositories, PostgresBackend,
     RedisBackend,
 };
 pub use config::DataLayerConfig;
+pub use database::{DatabaseDriver, SqlDatabaseConfig, SqlPoolConfig, DEFAULT_SQLITE_DATABASE_URL};
 pub use error::DataLayerError;
+pub use maintenance::{
+    DatabaseMaintenanceSummary, DatabasePoolSummary, StatsDailyAggregationInput,
+    StatsDailyAggregationSummary, StatsHourlyAggregationInput, StatsHourlyAggregationSummary,
+    WalletDailyUsageAggregationInput, WalletDailyUsageAggregationResult,
+};