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.

apps/aether-gateway/src/orchestration/effects.rs

@@ -93,7 +93,7 @@ pub(crate) enum LocalExecutionEffect<'a> {
 }
 
 struct PoolFeedbackContext {
-    runner: aether_data::redis::RedisKvRunner,
+    runner: aether_data::driver::redis::RedisKvRunner,
     pool_config: AdminProviderPoolConfig,
     sticky_session_token: Option<String>,
 }