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/notifications.py

@@ -33,7 +33,7 @@ async def dispatch_notifications(
 async def _send_email(cfg: dict, alert: dict) -> dict:
     try:
         msg = EmailMessage()
-        msg["Subject"] = f"[Fabled Scryer] {alert['state']} — {alert['rule_name']}"
+        msg["Subject"] = f"[Roundtable] {alert['state']} — {alert['rule_name']}"
         msg["From"] = cfg.get("username", "roundtable@localhost")
         msg["To"] = ", ".join(cfg["recipients"])
         msg.set_content(