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/lib.rs

@@ -51,7 +51,6 @@ mod oauth;
 mod orchestration;
 mod provider_key_auth;
 pub(crate) use aether_provider_transport as provider_transport;
-mod query;
 mod rate_limit;
 mod request_candidate_runtime;
 mod router;