feat(settings): panel-tunable feature flags

Add a Feature Flags card to the Settings page that toggles env-gated
subsystems (external/internal PR review, web research, strategy engine,
pitch provisioning, RAG auto-update, transcript pruning) directly from
the panel instead of hand-editing environment variables.

Flags persist in system_settings as 'true'/'false' and are overlaid onto
the live config singleton at startup; an unset flag keeps its
environment/config default. A toggle takes effect on the next backend
restart — no per-consumer re-routing.

Backend: FEATURE_FLAGS registry + bool validator + get_bool accessor on
SettingsService; feature_flag_effective_values and
apply_persisted_feature_flags; GET /settings/feature-flags; best-effort
startup overlay in the app lifespan.

Frontend: settingsApi.getFeatureFlags / setFeatureFlag and a
FeatureFlagsCard rendered full-width below the settings grid.

panel/src/app/(dashboard)/settings/page.tsx

@@ -27,6 +27,7 @@ import {
 import { toast } from "sonner";
 import { API_URL, WS_URL } from "@/lib/constants";
 import { TranscriptRetentionCard } from "@/components/settings/transcript-retention-card";
+import { FeatureFlagsCard } from "@/components/settings/feature-flags-card";
 
 export default function SettingsPage() {
   const { theme, setTheme } = useTheme();
@@ -241,6 +242,10 @@ export default function SettingsPage() {
         </Card>
       </div>
 
+      {/* Feature Flags — master switches for optional subsystems (full width;
+          persisted server-side, applied on next restart). */}
+      <FeatureFlagsCard />
+
       {/* Save Button */}
       <div className="flex justify-end">
         <Button onClick={handleSave}>