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/Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+name = "aether-data-schema"
+version = "0.1.0"
+edition.workspace = true
+license.workspace = true
+repository.workspace = true
+description = "Logical schema parser and SQL generator for Aether data storage"
+
+[dependencies]
+clap = { version = "4", features = ["derive"] }
+serde = { workspace = true, features = ["derive"] }
+thiserror.workspace = true
+toml = "0.8"