Add resource limits, abuse protection and overload handling

Running games always continue; new work is refused when there is no
capacity; abuse is rejected immediately.

- Docker: CPU/memory/pids caps per container (overridable in .env);
  uvicorn message size, queue and concurrency limits
- Server abuse limits: connections total/per IP (close 1013), message
  rate per connection (token bucket, close 1008 on floods), lobby
  creation rate per IP
- Load monitor: event loop lag, CPU of the own cgroup quota and
  throttling, with hysteresis (normal/busy/overloaded, severe lag
  overloads immediately)
- Admission control: create_lobby and start_game refused while
  overloaded or MAX_ACTIVE_GAMES are running; server_status message
  broadcast on changes; /health reports load but stays ok
- AI bridge: timeout sent with requests, in-flight cap, circuit breaker
- AI service: bounded workers and queue ("overloaded"), drops expired
  requests, degrades to the heuristic agent when busy or too slow
- Website: busy notices, Create/Start disabled when not accepting
  games, slow retry when the server is full
- Tests for limiters, load levels, admission control and AI service
  limits; docs updated

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

wizard_server/tests/helpers.py

@@ -0,0 +1,33 @@
+"""Helpers shared by the server tests."""
+
+SECRET = "test-secret"
+
+# Tests play much faster than humans, so the message rate limit is raised.
+TEST_SETTINGS = dict(
+    ai_shared_secret=SECRET,
+    ai_move_delay=0,
+    trick_pause=0,
+    ai_decision_timeout=2,
+    message_rate=10_000,
+    message_burst=10_000,
+)
+
+
+def receive_until(ws, predicate, limit=5000):
+    for _ in range(limit):
+        message = ws.receive_json()
+        if predicate(message):
+            return message
+    raise AssertionError("expected message not received")
+
+
+def is_state(message):
+    return message["type"] == "state"
+
+
+def connect(client, token=None):
+    ws = client.sockets.enter_context(client.websocket_connect("/ws"))
+    ws.send_json({"type": "hello", "token": token})
+    welcome = receive_until(ws, lambda m: m["type"] == "welcome")
+    receive_until(ws, is_state)
+    return ws, welcome