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/pixiv/api.rs

@@ -136,6 +136,12 @@ impl PixivAPI {
             .bearer_auth(access_token)
             .send()
             .await?;
+        if !response.status().is_success() {
+            return Err(PixivError::Api(format!(
+                "status {}",
+                response.status()
+            )));
+        }
         let json: serde_json::Value = serde_json::from_str(&response.text().await?)?;
         if json.get("error").is_some() {
             let message = json
@@ -186,6 +192,12 @@ impl PixivAPI {
             .bearer_auth(access_token)
             .send()
             .await?;
+        if !response.status().is_success() {
+            return Err(PixivError::Api(format!(
+                "status {}",
+                response.status()
+            )));
+        }
         let json: serde_json::Value = serde_json::from_str(&response.text().await?)?;
         if json.get("error").is_some() {
             let message = json