docs: roundtable rename sweep

Sweeps README, docs/, example configs, code docstrings, report and
notification subject lines, plugin catalog URL default, and stray
comments in Dockerfile/entrypoint.sh/.gitignore. Adds a legacy note
to docs/core/configuration.md explaining the FABLEDSCRYER_* fallback.
docker-compose DB credentials intentionally left as fabledscryer to
preserve the existing pgdata volume.

roundtable/cli.py

@@ -9,6 +9,6 @@ from .app import create_app
 @click.option("--port", default=5000, type=int)
 @click.option("--debug", is_flag=True, default=False)
 def main(config: str, host: str, port: int, debug: bool) -> None:
-    """Start the Fabled Scryer server."""
+    """Start the Roundtable server."""
     app = create_app(config_path=Path(config))
     app.run(host=host, port=port, debug=debug)