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/overrides/README.md

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+# Schema Overrides
+
+This directory is reserved for rare driver-specific SQL that cannot be
+represented by `schema/logical/*.toml` yet.
+
+It is not a normal schema source tree. Leave it with this README only until a
+real override exists.
+
+Use overrides for:
+
+- Postgres-only indexes such as GIN/GiST expression indexes
+- views
+- custom trigger/function SQL
+- dialect-specific bootstrap details
+- temporary compatibility SQL while a domain is being moved to logical schema
+
+Do not place ordinary table/column/index definitions here when the logical
+schema generator can express them.
+
+Do not add `.gitkeep` placeholder directories. When an override is introduced,
+create only the directory and SQL file that are actually needed, then add that
+file to the relevant source manifest so `compose_schema.sh check` covers it.