feat(settings): add public_base_url helper with request.host_url fallback

roundtable/core/settings.py

@@ -202,3 +202,27 @@ def load_settings_sync(db_url: str) -> dict[str, Any]:
         if key.startswith("plugin.") and key not in out:
             out[key] = value
     return out
+
+
+# ─────────────────────────────────────────────────────────────────────────────
+# External URL helper
+# ─────────────────────────────────────────────────────────────────────────────
+
+def public_base_url(request) -> str:
+    """Return the externally-reachable base URL for this Roundtable instance.
+
+    Prefers the admin-configured 'general.public_base_url' setting (cached in
+    current_app.config['PUBLIC_BASE_URL']) when set. Falls back to
+    request.host_url stripped of its trailing slash, so unconfigured
+    single-hostname installs keep working with zero config.
+
+    Use this — NOT request.host_url — anywhere you build a URL that will be
+    consumed by something outside this Quart request: install scripts, share
+    links, alert notifications, webhook callbacks. The Host header is not
+    reliable behind proxies or on multi-hostname deployments.
+    """
+    from quart import current_app
+    configured = (current_app.config.get("PUBLIC_BASE_URL") or "").strip()
+    if configured:
+        return configured.rstrip("/")
+    return request.host_url.rstrip("/")