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/src/repository/provider_catalog/mod.rs

@@ -1,5 +1,7 @@
 mod memory;
-mod sql;
+mod mysql;
+mod postgres;
+mod sqlite;
 
 #[allow(unused_imports)]
 pub(crate) use aether_data_contracts::repository::provider_catalog::{
@@ -8,4 +10,6 @@ pub(crate) use aether_data_contracts::repository::provider_catalog::{
     StoredProviderCatalogKeyPage, StoredProviderCatalogKeyStats, StoredProviderCatalogProvider,
 };
 pub use memory::InMemoryProviderCatalogReadRepository;
-pub use sql::SqlxProviderCatalogReadRepository;
+pub use mysql::MysqlProviderCatalogReadRepository;
+pub use postgres::SqlxProviderCatalogReadRepository;
+pub use sqlite::SqliteProviderCatalogReadRepository;

crates/aether-data/src/repository/provider_catalog/postgres.rs

@@ -2525,7 +2525,7 @@ fn map_key_row(row: &PgRow) -> Result<StoredProviderCatalogKey, DataLayerError>
 #[cfg(test)]
 mod tests {
     use super::SqlxProviderCatalogReadRepository;
-    use crate::postgres::{PostgresPoolConfig, PostgresPoolFactory};
+    use crate::driver::postgres::{PostgresPoolConfig, PostgresPoolFactory};
 
     #[tokio::test]
     async fn repository_constructs_from_lazy_pool() {
@@ -2567,7 +2567,7 @@ mod tests {
             assert!(sql.contains("concurrent_limit"));
         }
 
-        let source = include_str!("sql.rs");
+        let source = include_str!("postgres.rs");
         assert!(source.contains("concurrent_limit,"));
         assert!(source.contains("concurrent_limit = $13"));
         assert!(source.contains(".bind(key.concurrent_limit)"));
@@ -2577,7 +2577,7 @@ mod tests {
     #[test]
     fn provider_api_keys_concurrent_limit_schema_is_nullable_without_default() {
         let migration = include_str!(
-            "../../../migrations/20260502000000_add_provider_key_auth_channel_mismatch_formats.sql"
+            "../../../migrations/postgres/20260502000000_add_provider_key_auth_channel_mismatch_formats.sql"
         );
         let concurrent_limit_line = migration
             .lines()
@@ -2589,7 +2589,7 @@ mod tests {
             "add column if not exists concurrent_limit integer;"
         );
 
-        let baseline = include_str!("../../../bootstrap/20260413020000_baseline_v2.sql");
+        let baseline = crate::lifecycle::bootstrap::postgres::EMPTY_DATABASE_SNAPSHOT_SQL;
         assert!(baseline.contains("CREATE TABLE IF NOT EXISTS public.provider_api_keys"));
         assert!(baseline.contains("concurrent_limit integer,"));
     }
@@ -2603,7 +2603,7 @@ mod tests {
             assert!(sql.contains("allow_auth_channel_mismatch_formats"));
         }
 
-        let source = include_str!("sql.rs");
+        let source = include_str!("postgres.rs");
         assert!(
             source
                 .matches("auth_type_by_format,\n  allow_auth_channel_mismatch_formats,\n  api_key",)