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

backend/app/routers/worlds.py

@@ -12,6 +12,7 @@ from sqlalchemy.orm import selectinload
 from app.database import get_db
 from app.dependencies import CurrentUser
 from app.models.world import World, WorldVisibility
+from app.services.game_servers import supervisor
 
 router = APIRouter()
 
@@ -106,6 +107,40 @@ async def get_world(
     return world
 
 
+class JoinResponse(BaseModel):
+    """Address of the dedicated server hosting this world."""
+    world_id: int
+    host: str
+    port: int
+
+
+@router.post("/{world_id}/join", response_model=JoinResponse)
+async def join_world(
+    world_id: int,
+    current_user: CurrentUser,
+    db: DbDep,
+):
+    """Ensure a dedicated game server is running for this world and return its
+    ENet address for the client to connect to.
+
+    503 if game-server orchestration isn't enabled on this backend (e.g. a
+    plain dev/test instance with no Godot binary).
+    """
+    result = await db.execute(select(World).where(World.id == world_id))
+    world = result.scalar_one_or_none()
+    if not world:
+        raise HTTPException(status_code=404, detail="World not found")
+    if world.visibility != WorldVisibility.PUBLIC and world.owner_id != current_user.id:
+        raise HTTPException(status_code=404, detail="World not found")
+
+    try:
+        info = await supervisor.ensure_server(str(world_id))
+    except RuntimeError as exc:
+        raise HTTPException(status_code=503, detail=str(exc))
+
+    return JoinResponse(world_id=world_id, host=info["host"], port=info["port"])
+
+
 @router.post("/", response_model=WorldResponse, status_code=201)
 async def create_world(
     world_data: WorldCreate,