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>

.gitignore

@@ -1,2 +1,8 @@
 web/node_modules/
 web/dist/
+node_modules/
+dist/
+data/
+*.db
+*.db-wal
+*.db-shm