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>

client/src/components/Header.tsx

@@ -37,6 +37,11 @@ export default function Header() {
           </Link>
         )}
         {user?.id && typeof user.id === "number" && <NotificationPanel />}
+        {user?.elo != null && typeof user.id === "number" && (
+          <Link href={`/user/${user.name}`} className="btn btn-ghost btn-sm font-mono tabular-nums" title="Deine ELO-Wertung">
+            {user.elo}
+          </Link>
+        )}
         <ThemeToggle />
         <label tabIndex={0} htmlFor="auth-modal" className="btn btn-ghost btn-circle avatar">
           <div className="w-10 rounded-full">