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/bootstrap/postgres/manifest.txt

@@ -0,0 +1,10 @@
+001_types_and_tables.sql
+002_defaults.sql
+003_constraints.sql
+004_indexes.sql
+005_foreign_keys.sql
+006_footer.sql
+100_usage_capture.sql
+110_redeem_codes.sql
+120_stats_rollups.sql
+130_stats_cost_savings.sql