PC3 Phase 2.2: game-server orchestration + POST /api/worlds/{id}/join

- backend/app/services/game_servers.py: an in-process supervisor. One
  headless Godot dedicated-server process per world, keyed by world_id,
  spawned on demand from the join endpoint, port-allocated from a
  configured range, with a background reaper that kills a world's server
  once its status file reports 0 players for the idle timeout. Off by
  default (GAME_SERVERS_ENABLED); the checkpoint/playtest launcher enables it.
- POST /api/worlds/{id}/join -> {world_id, host, port}; 404 for
  unknown/hidden worlds, 503 when orchestration is disabled. Auth required.
- main.py lifespan starts/stops the supervisor.
- config.py: GODOT_BIN, GAME_PROJECT_DIR, GAME_SERVER_HOST, port range,
  idle timeout.
- tests/test_world_join.py (auth / 404 / visibility / 503) -- suite 72 passed.
- checkpoint runner also runs the 2p multiplayer shell test.

Verified end to end: enabled orchestration, POST /join spawned the Godot
server, a real client connected to the returned address and moved; backend
shutdown reaped the process.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015jtxAiu5dAW4WxCNdGkrkJ

scripts/run_checkpoint_tests.py

@@ -156,6 +156,16 @@ def main():
                             except subprocess.TimeoutExpired:
                                 game_server.kill()
                 results["godot smoke tests"] = godot_ok
+
+                # 2-player multiplayer test (its own shell harness: server + 2
+                # real client processes).
+                mp_script = ROOT / "scripts" / "run_multiplayer_2p_test.sh"
+                if mp_script.exists():
+                    print("\n--- scripts/run_multiplayer_2p_test.sh ---")
+                    mp_env = os.environ.copy()
+                    mp_env["GODOT_BIN"] = godot_bin
+                    r = run(["bash", str(mp_script)], env=mp_env)
+                    results["multiplayer 2p test"] = r.returncode == 0
         finally:
             server_proc.terminate()
             try: