Wire browse-worlds screen to GET /api/worlds

- main_menu.gd: Play button now navigates to world_list.tscn
- world_list.gd: fix invalid theme_override_constants property-path syntax
  (use add_theme_constant_override), fix @onready node paths missing
  /HeaderHBox/, cast Variant dict values to String to avoid type-inference
  errors when cards actually render
- world_list.tscn: wire up Back/Refresh button signal connections (scene
  had zero [connection] blocks, buttons were dead)
- backend: fix real bug where World.visibility enum sent member names
  ("PUBLIC") to Postgres instead of lowercase values, breaking both GET
  and POST /api/worlds/ for any real query
- backend: fix WorldResponse.created_at typed as str instead of datetime,
  causing a second 500 on GET once the enum bug was fixed
- add smoke_test_world_list.gd: headless scene-level smoke test that
  registers a user, instantiates the real scene, and confirms world cards
  render from live backend data (not just an API-level check)

backend/app/models/world.py

@@ -21,7 +21,12 @@ class World(Base):
     description = Column(Text, nullable=True)
     owner_id = Column(Integer, ForeignKey("users.id"), nullable=False)
     visibility = Column(
-        ENUM(WorldVisibility, name="world_visibility", create_type=False),
+        ENUM(
+            WorldVisibility,
+            name="world_visibility",
+            create_type=False,
+            values_callable=lambda enum_cls: [e.value for e in enum_cls],
+        ),
         default=WorldVisibility.PUBLIC,
     )
     created_at = Column(DateTime(timezone=True), server_default=func.now())