Update provider seeding logic and documentation to allow creation of admin/shared providers without requiring an API key. Adjust environment variable handling so that a blank or missing API_KEY is valid when API_BASE is set, enabling support for upstream TTS providers that do not require authentication. Remove defaulting to 'none' for API keys in API routes and ensure headers are omitted when no key is present. Add and update tests to verify correct handling of keyless providers.
BREAKING CHANGE: Providers can now be seeded without an API key; API_KEY is no longer required if API_BASE is set. Existing deployments relying on a non-empty API_KEY for seeding should review their environment configuration.
Add a dedicated Admin Panel documentation page detailing management of shared TTS providers and site features via the admin UI. Update all TTS provider guides, environment variable references, and deployment docs to clarify the new runtime configuration model: environment variables serve as first-boot seeds only, with ongoing management handled through the admin interface. Revise sidebars and cross-links to include the new admin panel docs and clarify the distinction between legacy env-based and admin-managed configuration.
