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.

Dockerfile

@@ -26,7 +26,7 @@ RUN chmod +x /entrypoint.sh
 # Runtime directories — owned by app user.
 # The container runs as root so the entrypoint can start NUT daemons (USB
 # access requires root); it drops to 'app' (uid 1000) via gosu before
-# launching fabledscryer.
+# launching roundtable.
 RUN useradd -m -u 1000 app \
     && mkdir -p /data/playbook_cache /data/plugins \
     && chown -R app:app /data /app