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/lib/api/settings.ts

@@ -4,6 +4,17 @@ export interface SettingsResponse {
   settings: Record<string, string>;
 }
 
+export interface FeatureFlag {
+  key: string;
+  label: string;
+  enabled: boolean;
+}
+
+export interface FeatureFlagsResponse {
+  flags: FeatureFlag[];
+  note: string;
+}
+
 export const settingsApi = {
   // GET /api/settings — all runtime-editable settings as a flat key→value map.
   getAll: async (): Promise<Record<string, string>> => {
@@ -15,4 +26,13 @@ export const settingsApi = {
     const { data } = await api.put<SettingsResponse>(`/settings/${key}`, { value });
     return data.settings;
   },
+  // GET /api/settings/feature-flags — effective flag values (override, else env).
+  getFeatureFlags: async (): Promise<FeatureFlagsResponse> => {
+    const { data } = await api.get<FeatureFlagsResponse>("/settings/feature-flags");
+    return data;
+  },
+  // PUT /api/settings/{key} — persist a feature flag as "true"/"false".
+  setFeatureFlag: async (key: string, enabled: boolean): Promise<void> => {
+    await api.put<SettingsResponse>(`/settings/${key}`, { value: enabled ? "true" : "false" });
+  },
 };