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.
