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/reports.py

@@ -152,7 +152,7 @@ def _render_report_text(data: dict) -> str:
     now = data["generated_at"]
     lines: list[str] = []
 
-    lines.append("Fabled Scryer — Weekly Report")
+    lines.append("Roundtable — Weekly Report")
     lines.append(f"Generated: {now.strftime('%Y-%m-%d %H:%M UTC')}")
     lines.append("")
 
@@ -204,7 +204,7 @@ def _render_report_text(data: dict) -> str:
         lines.append("")
 
     lines.append("─" * 40)
-    lines.append("Fabled Scryer — https://git.fabledsword.com/bvandeusen/FabledScryer")
+    lines.append("Roundtable — https://git.fabledsword.com/bvandeusen/Roundtable")
     return "\n".join(lines)
 
 
@@ -222,7 +222,7 @@ async def send_report(app) -> tuple[bool, str]:
 
     body = _render_report_text(data)
     date_str = data["generated_at"].strftime("%Y-%m-%d")
-    subject = f"[Fabled Scryer] Weekly Report — {date_str}"
+    subject = f"[Roundtable] Weekly Report — {date_str}"
 
     msg = EmailMessage()
     msg["Subject"] = subject