fix(retry): let slow downloads finish, and never re-run a finished task

P1 of the retry audit, from the report's "reliability and diagnosis" batch.

- Media downloads no longer share the 30s *total* timeout of metadata
  fetches. The size caps allowed 10 MiB (reupload fallback) and 512 MiB
  (ugoira frame zip) while the clock allowed 30s, so a slow link made those
  posts impossible: `MEDIA_CLIENT` has no total timeout and instead bounds
  the response head and every chunk with a 30s *idle* window, which keeps the
  stalled-connection protection. Verified against a local probe: the old
  policy aborts a 40s download at 30.0s, the new one completes it (2 MiB,
  40.1s), and a body that stops delivering still fails after exactly 30s.
- A finished row's write-back is no longer best-effort. `delete_row` failing
  left the row `in_progress` with a live lease, so the next sweep flipped it
  back to `pending` and re-ran a completed task — a second album, a second
  prompt, a second channel copy. Both terminal writes are now retried, and a
  delete that still fails falls back to a `done` tombstone that neither the
  lease query nor the sweep looks at; reschedule (no safe tombstone: marking
  it done would drop the retry silently) logs what the sweep will do.
- bsky and pixiv no longer present a *failed* video conversion as a post with
  no media: the remux/ugoira error propagates (pixiv keeps its retry class,
  bsky reports Transient), so the user sees the real cause and `fetch` gets
  its retries. bsky's "no ffmpeg" case stays a degradation — retrying a
  deployment gap cannot help.
- pixiv's token exchange checks the HTTP status before parsing the body, so a
  429/5xx from the OAuth endpoint stays retryable instead of becoming a
  permanent Api/Json error (via the shared `pixiv_error_is_retryable`), and
  startup validation only disables pixiv for a rejected credential — one 503
  while the container came up used to turn every later pixiv link into
  "pixiv support is disabled".

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

@@ -76,6 +76,13 @@ impl PixivAPI {
             .header("User-Agent", AUTH_USER_AGENT)
             .send()
             .await?;
+        // Check the status *before* reading the body: a 429/5xx from the
+        // token endpoint is worth retrying (the class comes from
+        // `is_retryable`), while parsing a maintenance page as JSON turned it
+        // into a permanent `Api`/`Json` error with no retry at all.
+        if !response.status().is_success() {
+            return Err(PixivError::Status(response.status().as_u16()));
+        }
         let json: serde_json::Value = serde_json::from_str(&response.text().await?)?;
         let access_token = json
             .get("access_token")
@@ -134,8 +141,11 @@ impl PixivAPI {
         let mut illustration = Illustration::from_model(&model);
         if matches!(&model.r#type, TypeModel::Ugoira) {
             // Real ugoira support: download the frame zip and encode an MP4.
-            // Without ffmpeg (or on encode failure) the post stays
-            // unsupported (empty media, like Python).
+            // Without ffmpeg the post stays unsupported (empty media, like
+            // Python) — but a *failed* download/encode is reported instead:
+            // a ugoira post has no static image to fall back to, so
+            // swallowing it would present a transient zip-download error as
+            // "this post has no media", with the retries skipped.
             match self.ugoira_video(illust_id).await {
                 Ok(Some((mp4_path, _keep_alive))) => {
                     illustration.media.push(Media::Video {
@@ -146,7 +156,10 @@ impl PixivAPI {
                     illustration._keep_alive = Some(_keep_alive);
                 }
                 Ok(None) => {}
-                Err(e) => log::error!("ugoira encode failed for {illust_id}: {e}"),
+                Err(e) => {
+                    log::error!("ugoira encode failed for {illust_id}: {e}");
+                    return Err(FetchError::Pixiv(e));
+                }
             }
         }
         Ok(illustration)