SSE endpoint publishing quest-completed transitions (task 4d7bae00)

The panel needs live transitions, so add ONE in-process publisher
(api/events.py, installed on app.state.publisher in the lifespan) and
a GET /api/v1/panel/events SSE route on the panel router — it inherits
the single require_panel_token check. The complete route now builds
the event through the core builder and publishes it only when the
core's .appended flag says an actual transition happened; a no-op
re-complete publishes nothing. Tests stream from a real uvicorn
instance on an ephemeral port because httpx's ASGITransport buffers
endless SSE bodies.

api/nestquest_core.py

@@ -116,6 +116,10 @@ core_snapshot = importlib.import_module(f"{_CORE_MODULE_NAME}.snapshot")
 #: ``uncomplete_instance``) the panel's write routes call.
 core_completion = importlib.import_module(f"{_CORE_MODULE_NAME}.completion")
 
+#: The transition-event builders (``core.events.build_quest_completed_event``
+#: et al.) the panel's write routes build their SSE events with.
+core_events = importlib.import_module(f"{_CORE_MODULE_NAME}.events")
+
 #: The explicit settings object (``core.settings.NestQuestSettings``)
 #: the snapshot builder's ``settings`` parameter takes.
 core_settings = importlib.import_module(f"{_CORE_MODULE_NAME}.settings")
@@ -124,6 +128,7 @@ __all__ = [
     "core",
     "core_completion",
     "core_db",
+    "core_events",
     "core_migrations",
     "core_snapshot",
     "core_settings",