fix: add ok check for singleflight type assertion in model probe

singleflight.Group.Do() returns any, which is type-asserted as bool
without an ok check at model_status.go:211. If a non-bool value is
returned (e.g. nil from shared/cache corruption), this panics.

Add ok check and return false (model probe failed) as a safe default.

web/backend/api/model_status.go

@@ -208,7 +208,10 @@ func (s *modelProbeCacheState) probe(cacheKey string, probeFunc func() bool) boo
 		return result, nil
 	})
 
-	result, _ := v.(bool)
+	result, ok := v.(bool)
+	if !ok {
+		return false
+	}
 	return result
 }