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/migrations/mysql/20260504000000_add_audit_logs.sql

@@ -0,0 +1,18 @@
+CREATE TABLE IF NOT EXISTS audit_logs (
+    id VARCHAR(64) PRIMARY KEY,
+    event_type VARCHAR(64) NOT NULL,
+    user_id VARCHAR(64),
+    api_key_id VARCHAR(64),
+    description TEXT NOT NULL,
+    ip_address VARCHAR(64),
+    user_agent VARCHAR(512),
+    request_id VARCHAR(128),
+    event_metadata TEXT,
+    status_code INT,
+    error_message TEXT,
+    created_at BIGINT NOT NULL,
+    KEY audit_logs_created_at_idx (created_at),
+    KEY audit_logs_event_type_idx (event_type),
+    KEY audit_logs_request_id_idx (request_id),
+    KEY audit_logs_user_id_idx (user_id)
+);