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/tests/architecture/mod.rs

@@ -23,6 +23,10 @@ pub(super) fn assert_no_sqlx_queries(root_relative_path: &str) {
     let patterns = [
         "sqlx::query(",
         "sqlx::query_scalar",
+        "sqlx::postgres::PgRow",
+        "sqlx::Row",
+        "PostgresPoolFactory",
+        "PostgresPool",
         "query_scalar::<",
         "QueryBuilder<",
     ];