feat(elo): add ELO rating system

- DB: elo column on user table (default 1200, added via ALTER TABLE IF NOT EXISTS)
- ELO calculated after every game save (K=32, min 100)
- Bots use their fixed ELO values; only human players' ELO changes
- AI page games save with bot name (e.g. 'Holzpferd Heinz') instead of 'Stockfish'
- User profile shows ELO badge below name
- Header shows current ELO for logged-in users (links to profile)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/src/controllers/auth.controller.ts

@@ -223,6 +223,7 @@ export const loginUser = async (req: Request, res: Response) => {
             wins: users[0].wins,
             losses: users[0].losses,
             draws: users[0].draws,
+            elo: users[0].elo,
             role: users[0].role
         };
         req.session.save(() => {