feat: add SQLite and MySQL database support with setup wizard selection (RCA-21)

Replace in-memory storage with a database abstraction layer supporting SQLite
and MySQL. Users choose their preferred database during the first-time setup
wizard. The server persists the database config to data/db-config.json and
loads it automatically on restart.

- Add database abstraction interfaces (ICookieStore, IDeviceStore, IAgentStore, IAdminStore)
- Implement SQLite driver using better-sqlite3 with WAL mode
- Implement MySQL driver using mysql2 connection pooling
- Keep memory-backed driver for backwards compatibility and testing
- Add database selection step (step 2) to the setup wizard UI
- Update setup API to accept dbConfig and initialize the chosen database
- Update RelayServer to use async store interfaces with runtime store replacement

Co-Authored-By: Paperclip <noreply@paperclip.ing>

src/cli.ts

@@ -1,21 +1,39 @@
 import { RelayServer } from "./relay/index.js";
+import { loadDbConfig, createStores } from "./relay/db/index.js";
 
 const port = parseInt(process.env.PORT ?? "8080", 10);
 const host = process.env.HOST ?? "0.0.0.0";
 
-const server = new RelayServer({ port, host });
+async function main() {
+  // Load saved database config (if any)
+  const dbConfig = loadDbConfig();
+  let stores;
+  if (dbConfig) {
+    console.log(`Loading ${dbConfig.type} database...`);
+    stores = await createStores(dbConfig);
+  } else {
+    console.log("No database configured — starting with in-memory storage.");
+    console.log("Configure a database during setup to enable persistent storage.");
+  }
 
-server.start().then(() => {
+  const server = new RelayServer({ port, host, stores });
+
+  await server.start();
   console.log(`CookieBridge relay server listening on ${host}:${port}`);
-});
 
-process.on("SIGINT", async () => {
-  console.log("\nShutting down...");
-  await server.stop();
-  process.exit(0);
-});
+  process.on("SIGINT", async () => {
+    console.log("\nShutting down...");
+    await server.stop();
+    process.exit(0);
+  });
 
-process.on("SIGTERM", async () => {
-  await server.stop();
-  process.exit(0);
+  process.on("SIGTERM", async () => {
+    await server.stop();
+    process.exit(0);
+  });
+}
+
+main().catch((err) => {
+  console.error("Failed to start server:", err);
+  process.exit(1);
 });