M1: backend (Express+ws+better-sqlite3) + integration tests

- server/db.js: SQLite schema mirroring Firestore doc tree
- server/handlers.js: action -> shared turn fn -> tx persist -> broadcast
- server/index.js: REST endpoints + WebSocket real-time push
- server/server.test.js: 7 integration tests (REST CRUD + combat flow)
- --forceExit for jest (open WS handles)

Backend boots, serves state, persists to SQLite.

server/package.json

@@ -7,7 +7,7 @@
   "scripts": {
     "dev": "node --watch index.js",
     "start": "node index.js",
-    "test": "jest"
+    "test": "jest --forceExit"
   },
   "dependencies": {
     "@ttrpg/shared": "*",