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.

entrypoint.sh

@@ -1,5 +1,5 @@
 #!/bin/sh
-# FabledScryer container entrypoint.
+# Roundtable container entrypoint.
 #
 # When NUT_MANAGED=1:
 #   - Generates /etc/nut/*.conf from environment variables