Migrate Socket.IO server to ASGI, fix bugs, harden, add socket tests

Replace Flask-SocketIO + eventlet with python-socketio AsyncServer on an
ASGI app served by uvicorn (Python 3.14). The server is no longer started
as an import side-effect; `python -m app` runs uvicorn for dev and the
Docker image runs `uvicorn api:app`.

Bug fixes:
- create_game now mints a real uuid gid and returns it to the creator
  (was hardcoded 'a').
- play_card resolves the player's hand and plays the selected Card (was
  indexing a method and crashing).

Hardening:
- Identity binding: every action derives the seat from the connection
  (sid -> {gid, order}); clients no longer pass a player number, closing
  the hidden-cards cheat where any client could request any hand.
- Secure token-based reconnect (per-player secret token).
- disconnect handler marks players offline and drops empty games (no
  more leaked games), notifying the room via player_connection.
- Guards for unknown gid, double start_game, and bad input; engine
  exception messages are forwarded instead of swallowed.
- Lobby payload is public-only (no sids/tokens); game_status carries a
  completed flag.
- /health endpoint via other_asgi_app; env-driven CORS and logging.

Infra:
- Dockerfile -> python:3.14-slim, uvicorn CMD, drop dead venv lines.
- requirements.txt -> python-socketio/engineio + uvicorn; drop eventlet,
  Flask-SocketIO, Flask-Session.
- docker-compose: drop unused debugpy port and obsolete version key.
- Remove redundant start.py; gitignore /.venv.

Tests: test_socket.py drives the handlers (identity binding, lobby
privacy, reconnect, disconnect cleanup, error handling, play flow).
Full suite: 29 passing.

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

Dockerfile

@@ -1,24 +1,16 @@
-FROM python:3.8-slim-buster
+FROM python:3.14-slim
 
 WORKDIR /app
-RUN python3 -m venv venv
-RUN . venv/bin/activate
-COPY requirements.txt requirements.txt
-RUN pip3 install -r requirements.txt
 
-# Debug image reusing the base
-# Install dev dependencies for debugging
-RUN pip install debugpy
-# Keeps Python from generating .pyc files in the container
-ENV PYTHONDONTWRITEBYTECODE 1
-# Turns off buffering for easier container logging
-ENV PYTHONUNBUFFERED 1
+# Keeps Python from generating .pyc files and turns off output buffering.
+ENV PYTHONDONTWRITEBYTECODE=1 \
+    PYTHONUNBUFFERED=1
+
+COPY requirements.txt requirements.txt
+RUN pip install --no-cache-dir -r requirements.txt
 
 COPY . .
-ENV FLASK_ENV=development
-# ENTRYPOINT ["python3"]
-CMD ["python3", "-m", "app"]
-# For start Jakub version run script
-# CMD ["python3", "-m", "flask", "run", "-h", "0.0.0.0", "-p", "5000" ]
 
-# , "-m", "debugpy", "--listen", "0.0.0.0:5678", "-m", "app",  "--wait-for-client", "--multiprocess",
+EXPOSE 5000
+# Serve the ASGI Socket.IO app with uvicorn.
+CMD ["uvicorn", "api:app", "--host", "0.0.0.0", "--port", "5000"]