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/generated/mysql/baseline/004_proxy_nodes.sql

@@ -0,0 +1,45 @@
+-- Generated by aether-data-schema from schema/logical/*.toml.
+-- Do not edit generated files directly; edit logical schema or explicit overrides instead.
+
+CREATE TABLE IF NOT EXISTS proxy_nodes (
+    `id` VARCHAR(64) NOT NULL,
+    `name` VARCHAR(255) NOT NULL,
+    `ip` VARCHAR(512) NOT NULL,
+    `port` INT NOT NULL,
+    `region` VARCHAR(100),
+    `status` VARCHAR(32) NOT NULL DEFAULT 'online',
+    `registered_by` VARCHAR(64),
+    `last_heartbeat_at` BIGINT,
+    `heartbeat_interval` INT NOT NULL DEFAULT 30,
+    `active_connections` INT NOT NULL DEFAULT 0,
+    `total_requests` BIGINT NOT NULL DEFAULT 0,
+    `avg_latency_ms` DOUBLE,
+    `is_manual` TINYINT(1) NOT NULL DEFAULT 0,
+    `proxy_url` VARCHAR(500),
+    `proxy_username` VARCHAR(255),
+    `proxy_password` VARCHAR(500),
+    `created_at` BIGINT NOT NULL,
+    `updated_at` BIGINT NOT NULL,
+    `remote_config` JSON,
+    `config_version` INT NOT NULL DEFAULT 0,
+    `hardware_info` JSON,
+    `estimated_max_concurrency` INT,
+    `tunnel_mode` TINYINT(1) NOT NULL DEFAULT 0,
+    `tunnel_connected` TINYINT(1) NOT NULL DEFAULT 0,
+    `tunnel_connected_at` BIGINT,
+    `failed_requests` BIGINT NOT NULL DEFAULT 0,
+    `dns_failures` BIGINT NOT NULL DEFAULT 0,
+    `stream_errors` BIGINT NOT NULL DEFAULT 0,
+    `proxy_metadata` JSON,
+    PRIMARY KEY (`id`)
+);
+
+CREATE TABLE IF NOT EXISTS proxy_node_events (
+    `id` BIGINT NOT NULL AUTO_INCREMENT,
+    `node_id` VARCHAR(64) NOT NULL,
+    `event_type` VARCHAR(64) NOT NULL,
+    `detail` VARCHAR(500),
+    `created_at` BIGINT NOT NULL,
+    PRIMARY KEY (`id`)
+);
+