fix: Add retry logic for OpenRouter 429 rate limit errors

- Retries up to 3 times with exponential backoff (2s, 4s, 8s)
- Also handles 503 service unavailable errors
- Logs retry attempts for debugging
- Only retries rate limit errors, other errors fail immediately

src/lib/openrouter.ts

@@ -37,4 +37,4 @@ export function getOpenRouterClient(): OpenAI {
 }
 
 // Default OpenRouter model for vision tasks
-export const OPENROUTER_VISION_MODEL = 'google/gemma-3-27b-it';
+export const OPENROUTER_VISION_MODEL = 'google/gemma-3-27b-it:free';