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/drivers/sqlite/baseline/004_proxy_nodes.sql

@@ -0,0 +1,40 @@
+CREATE TABLE IF NOT EXISTS proxy_nodes (
+    id TEXT PRIMARY KEY,
+    name TEXT NOT NULL,
+    ip TEXT NOT NULL,
+    port INTEGER NOT NULL,
+    region TEXT,
+    status TEXT NOT NULL DEFAULT 'online',
+    registered_by TEXT,
+    last_heartbeat_at INTEGER,
+    heartbeat_interval INTEGER NOT NULL DEFAULT 30,
+    active_connections INTEGER NOT NULL DEFAULT 0,
+    total_requests INTEGER NOT NULL DEFAULT 0,
+    avg_latency_ms REAL,
+    is_manual INTEGER NOT NULL DEFAULT 0,
+    proxy_url TEXT,
+    proxy_username TEXT,
+    proxy_password TEXT,
+    created_at INTEGER NOT NULL,
+    updated_at INTEGER NOT NULL,
+    remote_config TEXT,
+    config_version INTEGER NOT NULL DEFAULT 0,
+    hardware_info TEXT,
+    estimated_max_concurrency INTEGER,
+    tunnel_mode INTEGER NOT NULL DEFAULT 0,
+    tunnel_connected INTEGER NOT NULL DEFAULT 0,
+    tunnel_connected_at INTEGER,
+    failed_requests INTEGER NOT NULL DEFAULT 0,
+    dns_failures INTEGER NOT NULL DEFAULT 0,
+    stream_errors INTEGER NOT NULL DEFAULT 0,
+    proxy_metadata TEXT
+);
+
+CREATE TABLE IF NOT EXISTS proxy_node_events (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    node_id TEXT NOT NULL,
+    event_type TEXT NOT NULL,
+    detail TEXT,
+    created_at INTEGER NOT NULL
+);
+