feat: add screenshot retry logic for transient browser failures

- Add isRetryableError() helper (retry on TimeoutError, Protocol error,
  Target closed, Session closed, Navigation failed, net::ERR_*)
- Wrap browser screenshot in retry loop (max 2 retries, exponential backoff)
- Add retryCount to ScreenshotResult, X-Retry-Count response header
- Validation/SSRF/auth errors are NOT retried
- 28 new tests (12 retry classification + 6 screenshot retry + route tests)

package-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "snapapi",
-  "version": "0.7.0",
+  "version": "0.8.0",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "snapapi",
-      "version": "0.7.0",
+      "version": "0.8.0",
       "dependencies": {
         "compression": "^1.8.1",
         "express": "^4.21.0",