refactor(data): remove MySQL and SQLite support

Use PostgreSQL as the only database backend across runtime, schema tooling, installation, Compose, and CI. Update regression tests and reject removed drivers explicitly.

crates/aether-data/runtime/src/driver/mod.rs

@@ -1,12 +1,7 @@
 //! Compatibility paths for database adapter crates.
 //!
-//! New adapter code belongs in `aether-data-postgres`, `aether-data-mysql`, or
-//! `aether-data-sqlite`. These modules preserve existing `aether_data::driver`
+//! Adapter code belongs in `aether-data-postgres`. These modules preserve existing `aether_data::driver`
 //! imports while application-facing composition remains in `backend`.
 
-#[cfg(feature = "mysql")]
-pub mod mysql;
 #[cfg(feature = "postgres")]
 pub mod postgres;
-#[cfg(feature = "sqlite")]
-pub mod sqlite;

crates/aether-data/runtime/src/driver/mysql.rs

@@ -1,3 +0,0 @@
-//! Compatibility facade for the MySQL adapter crate.
-
-pub use aether_data_mysql::*;

crates/aether-data/runtime/src/driver/sqlite.rs

@@ -1,3 +0,0 @@
-//! Compatibility facade for the SQLite adapter crate.
-
-pub use aether_data_sqlite::*;