Add persistence layer: TOTP auth, game history, restore

- db/ package: async SQLAlchemy engine + Player/Game/Guess models
- api/auth.py: passwordless TOTP login (pyotp), session token via socket auth
- api/history.py: record guesses/points, DB-backed standings, restore
  unfinished games on startup, host-only end_game
- api/__init__.py: auth-gated handlers, accounts map, rejoin via account
- frontend: Auth (QR + code) and History pages, resume/end-game in lobby/table
- docker-compose: real PostgreSQL service wired via DATABASE_URL
- tests_history.py for the persistence/auth layer; refresh CLAUDE.md

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

db/__init__.py

@@ -0,0 +1,9 @@
+"""Datova vrstva (persistencia). Nezavisla od Socket.IO/Flask, ako engine.
+
+Exportuje pripojenie a ORM modely; aplikacna logika (api/) ich pouziva.
+"""
+
+from db.db import Base, engine, async_session, init_db
+from db import models
+
+__all__ = ["Base", "engine", "async_session", "init_db", "models"]