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/schema/bootstrap/postgres/006_footer.sql

@@ -0,0 +1,10 @@
+-- Restore a normal lookup path before sqlx records this migration in the
+-- same transaction. sqlx inserts into `_sqlx_migrations` unqualified.
+SELECT pg_catalog.set_config('search_path', 'public', true);
+
+
+
+--
+-- PostgreSQL database dump complete
+--
+