site: classify HTTP status codes, make transient failures retryable

Twitter (syndication + auth GraphQL) mapped every non-2xx to NotFound,
killing retries on 429/5xx; bsky never checked status; pixiv network
errors arrived wrapped in PixivError and were excluded from the retry
loop. New FetchError::Transient covers 429/5xx from all sites, the
retry loop now also retries Pixiv errors, and 404/410 stay permanent.

crates/x-media/src/site/twitter/auth.rs

@@ -126,9 +126,14 @@ pub async fn fetch(id: &str) -> Result<Tweet, FetchError> {
         .header("referer", "https://x.com/")
         .send()
         .await?;
-    if !response.status().is_success() {
-        log::warn!("twitter auth fetch {id}: HTTP {}", response.status());
-        return Err(FetchError::NotFound);
+    // 404/410 = gone (permanent); 429/5xx = transient and retried by fetch.
+    let status = response.status();
+    if !status.is_success() {
+        log::warn!("twitter auth fetch {id}: HTTP {status}");
+        return match status.as_u16() {
+            404 | 410 => Err(FetchError::NotFound),
+            _ => Err(FetchError::Transient(format!("twitter auth status {status}"))),
+        };
     }
     let text = response.text().await?;
     let json: Value = serde_json::from_str(&text)?;