fix(pixiv): keep a frame-zip download hiccup retryable

download_media_to_file's non-transport errors all folded into PixivError::Api, and Api is permanent in pixiv_error_is_retryable — so one 429/5xx, stalled transfer or temp-file write failure while fetching the ugoira frame zip permanently failed the whole post, while the bot's own upload downloads classify the very same classes as retryable (classify_download_error). A new PixivError::Transient carries those classes into the existing retry policy (and into startup validation's 'stays enabled' branch); Http keeps its arm and everything else stays permanent. The retryable table and the startup-validation loop pin both halves.

crates/x-media/src/site/pixiv/api.rs

@@ -39,6 +39,11 @@ pub enum PixivError {
     Status(u16),
     #[error("pixiv api error: {0}")]
     Api(String),
+    /// A bad moment while preparing media: a transient download status
+    /// (429 / 5xx), a stalled transfer or a temp-file write failure. A retry
+    /// can change the answer, so the pixiv retry policy re-fetches these.
+    #[error("transient pixiv error: {0}")]
+    Transient(String),
 }
 
 /// Native pixiv app-API client.
@@ -228,6 +233,14 @@ impl PixivAPI {
             .await
             .map_err(|e| match e {
                 FetchError::Http(e) => PixivError::Http(e),
+                // A bad moment (429/5xx, a stalled transfer, a temp-file
+                // write failure) must stay retryable: folding it into Api
+                // made one hiccup permanently fail the whole ugoira post,
+                // while the bot's own upload downloads retry the same
+                // classes.
+                transient @ (FetchError::Transient(_) | FetchError::Io(_)) => {
+                    PixivError::Transient(format!("frame zip download failed: {transient}"))
+                }
                 other => PixivError::Api(format!("frame zip download failed: {other}")),
             })?;
         let frame_delays = metadata.frames.iter().map(|f| f.delay).collect::<Vec<_>>();