provider refactors and fixes

src/util/requester.rs

@@ -245,7 +245,10 @@ pub async fn post_json<S>(
 
         // If direct request failed, try FlareSolverr. Map its error to a Send+Sync error immediately,
         // so no non-Send error value lives across later `.await`s.
-        let flare_url = env::var("FLARE_URL").expect("FLARE_URL not set");
+        let flare_url = match env::var("FLARE_URL") {
+            Ok(url) => url,
+            Err(e) => return Err(format!("FLARE_URL not set: {e}").into()),
+        };
         let mut flare = Flaresolverr::new(flare_url);
         if self.proxy && env::var("BURP_URL").is_ok() {
             flare.set_proxy(true);