docs: make README consistent with implementation (#1)

## Summary

The README described two behaviors that don't match the actual code. This PR corrects both so the docs are accurate.

## Changes

- **CPU mode**: README claimed a *minimax* AI, but `public/game.js` (`cpuMove`) picks a random empty cell. Updated to say *random AI*.
- **SQLite**: README said `sql.js` runs *in-memory*, but `server/db.js` persists to a file (`server/data/games.db`). Updated to say *file-based persistence*.

No code changes — documentation only.

---------

Co-authored-by: robi <robi@alecsa.design>
Reviewed-on: #1
Reviewed-by: robi <robi@alecsa.design>

README.md

@@ -1,10 +1,10 @@
 # Tic Tac Toe
 
-A tic-tac-toe game with two modes: **CPU** (play against a minimax AI) and **Online** (real-time 2-player over WebSocket).
+A tic-tac-toe game with two modes: **CPU** (play against a CPU opponent that makes random moves) and **Online** (real-time 2-player over WebSocket).
 
 ## Stack
 - Frontend: vanilla JS + CSS (no build step)
-- Backend: Node.js + Express (serves static files) + `ws` (WebSocket) + `sql.js` (SQLite, in-memory)
+- Backend: Node.js + Express (serves static files) + `ws` (WebSocket) + `sql.js` (SQLite, file-based persistence)
 
 ## Project layout
 ```