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/generated/postgres/baseline/manifest.txt

@@ -0,0 +1,10 @@
+-- Generated by aether-data-schema from schema/logical/*.toml.
+-- Do not edit generated files directly; edit logical schema or explicit overrides instead.
+
+001_identity.sql
+002_provider_catalog.sql
+003_auth_config.sql
+004_proxy_nodes.sql
+005_wallet_billing.sql
+006_usage.sql
+007_stats.sql