Validate backendProviders against supported providers

## Problem The registry schema currently defines `backendProviders` as an array of arbitrary strings: ```ts backendProviders: z.array(z.string()).optional(), ``` However, the same file defines a list of supported backend providers: ```ts const backendProviderValues = ['next-auth', 'supabase', 'auth0'] ``` There is also an `isBackendProvider` type guard using this list, but the function is currently unused. ## Proposed change I propose using `backendProviderValues` directly in the Zod schema: ```ts const backendProviderValues = ['next-auth', 'supabase', 'auth0'] as const // ... backendProviders: z.array(z.enum(backendProviderValues)).optional(), ``` This would make the schema validate `backendProviders` against the supported provider values instead of accepting arbitrary strings. The unused `isBackendProvider` function could then be removed. I checked the repository and found that `backendProviders`, `backendProvidersCode`, `isBackendProvider`, and `backendProviderValues` are only referenced in this file. Would this validation change be desirable?