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/handlers/shared/mod.rs

@@ -25,7 +25,7 @@ pub(crate) use self::catalog::{
     encrypt_catalog_secret_with_fallbacks, masked_catalog_api_key, parse_catalog_auth_config_json,
     provider_catalog_key_supports_format, provider_key_health_summary,
     provider_key_status_snapshot_payload, sync_provider_key_oauth_status_snapshot,
-    sync_provider_key_quota_status_snapshot,
+    sync_provider_key_quota_status_snapshot, take_secret_prefix, take_secret_suffix,
 };
 pub(crate) use self::email_templates::{
     admin_email_template_definition, admin_email_template_html_key,