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/sqlite/20260504000000_add_audit_logs.sql

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