Buffer integration as settings tab + proper service labels

Follow-up to Phase 1b (#551). Two issues caught after deploy to staging:

1. Buffer missing from Integrations sidebar. The settings page at /settings/integrations/buffer/ was a standalone route with no entry in the sidebar nav. Users couldn't discover it. Existing pattern for integrations (Stripe, LINE, API Keys, Webhooks) is tabs within the main settings page, not separate routes.