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-contracts/Cargo.toml

@@ -9,6 +9,7 @@ description = "Shared data contracts and repository traits for Aether Rust servi
 [dependencies]
 aether-ai-formats.workspace = true
 async-trait.workspace = true
+chrono.workspace = true
 serde.workspace = true
 serde_json.workspace = true
 thiserror.workspace = true