Introduce `enableUserSignups` runtime setting to allow administrators to control
whether new accounts can be created. Update environment variable and documentation
references to support this feature. UI elements for account creation are now
conditionally rendered based on this flag. Signup attempts are blocked server-side
when disabled, including email, OAuth, and anonymous upgrades.
Add `assertUserSignupAllowed` utility for consistent enforcement and corresponding
unit tests to verify policy behavior.
Add database-backed runtime configuration for feature flags and TTS provider credentials, editable via a new admin UI panel. Replace static NEXT_PUBLIC_* and TTS provider env vars with admin-managed settings stored in the database and injected at SSR for client access. Implement admin-only panels for managing shared TTS provider credentials (with encrypted API keys) and live site feature flags. Add schema migrations, API routes, React contexts, and hooks for SSR-injected runtime config and live updates. Update client and server logic to resolve configuration from the database at runtime, enforcing admin restrictions and supporting migration from legacy env-based config.
BREAKING CHANGE: Feature flags and TTS provider credentials are now managed at runtime via the admin UI. Environment variables are only used for initial seeding and are ignored after first boot. Existing deployments must migrate configuration to the admin panel.
