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.

deploy/docker-compose.data.yml

@@ -41,6 +41,33 @@ services:
       retries: 5
     restart: unless-stopped
 
+  mysql:
+    image: ${MYSQL_IMAGE:-mysql:8.0}
+    container_name: aether-mysql
+    profiles:
+      - mysql
+    environment:
+      MYSQL_DATABASE: ${MYSQL_DATABASE:-aether}
+      MYSQL_USER: ${MYSQL_USER:-aether}
+      MYSQL_PASSWORD: ${MYSQL_PASSWORD:-aether}
+      MYSQL_ROOT_PASSWORD: ${MYSQL_ROOT_PASSWORD:-aether_root}
+      TZ: ${TZ:-Asia/Shanghai}
+    volumes:
+      - mysql_data:/var/lib/mysql
+    ports:
+      - "127.0.0.1:${MYSQL_PORT:-3306}:3306"
+    healthcheck:
+      test:
+        [
+          "CMD-SHELL",
+          "mysqladmin ping -h 127.0.0.1 -u$${MYSQL_USER} -p$${MYSQL_PASSWORD} --silent"
+        ]
+      interval: 5s
+      timeout: 5s
+      retries: 10
+    restart: unless-stopped
+
 volumes:
   postgres_data:
+  mysql_data:
   redis_data: