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>

package.json

@@ -22,13 +22,16 @@
   "license": "MIT",
   "type": "commonjs",
   "dependencies": {
+    "better-sqlite3": "^12.8.0",
     "jsonwebtoken": "^9.0.3",
+    "mysql2": "^3.20.0",
     "sodium-native": "^5.1.0",
     "typescript": "^5.9.3",
     "uuid": "^13.0.0",
     "ws": "^8.19.0"
   },
   "devDependencies": {
+    "@types/better-sqlite3": "^7.6.13",
     "@types/jsonwebtoken": "^9.0.10",
     "@types/node": "^25.5.0",
     "@types/sodium-native": "^2.3.9",