feat: add rateLimited and retryAfterMs to ProviderUsageData

New optional fields on ProviderUsageData allow providers to signal
rate-limit responses (e.g. HTTP 429) with a recommended backoff period.
The host app can use these to implement backoff scheduling and cache
stale data instead of showing errors during rate-limit windows.

Supports tokentopapp/tokentop#58

Author: nigel-dev

src/types/provider.ts

@@ -141,6 +141,10 @@ export interface ProviderUsageData {
   };
   fetchedAt: number;
   error?: string;
+  /** When true, the provider's API returned a rate-limit response (e.g. HTTP 429). */
+  rateLimited?: boolean;
+  /** Milliseconds the caller should wait before retrying. Parsed from Retry-After or a sensible default. */
+  retryAfterMs?: number;
 }
 
 // ---------------------------------------------------------------------------