Eliminate the showAllDeepInfraModels runtime/admin config and all code paths
that allowed restricting Deepinfra's model catalog. Deepinfra now always
shows the full model list regardless of API key or environment variable.
Update documentation, environment examples, admin panels, runtime config,
provider catalog logic, and tests to reflect this change.
Transition all TTS-related logic, types, and UI to use the new providerRef/providerType model in place of legacy ttsProvider fields. Introduce a centralized tts-provider-policy module to encapsulate provider/model capability checks, default value resolution, and compatibility logic. Update all API routes, contexts, hooks, components, and tests to use providerRef and providerType, ensuring consistent handling of built-in and shared TTS providers. Remove legacy defaultTtsModel config in favor of per-provider defaults and shared provider admin control. Add the showAllProviderModels runtime flag to restrict users to provider default models when desired.
BREAKING CHANGE: ttsProvider fields are replaced by providerRef/providerType throughout the codebase; defaultTtsModel config is removed in favor of per-provider defaults.
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.
