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/core/settings.py

@@ -50,7 +50,7 @@ DEFAULTS: dict[str, Any] = {
     "ansible.sources": [],
     "ping.threshold.good_ms": 50,
     "ping.threshold.warn_ms": 200,
-    "plugins.index_url": "https://git.fabledsword.com/bvandeusen/FabledScryer-plugins/raw/branch/main/index.yaml",
+    "plugins.index_url": "https://git.fabledsword.com/bvandeusen/Roundtable-plugins/raw/branch/main/index.yaml",
     # OIDC single-sign-on
     "oidc.enabled": False,
     "oidc.discovery_url": "",