fix(pixiv): capture ugoira frame timings in the post record (ordering bug)

The core writes the post record BEFORE extract_media, but the ugoira frame
delays were only memoized DURING extract_media — so write_post_record never saw
them and ugoira_frames was always empty in the record. Extract a memoized
_ugoira_meta (frames + zip url share ONE /v1/ugoira/metadata call regardless of
order) and inject client.fetch_ugoira_frames into the downloader (mirrors
Patreon's content_fetcher) so write_post_record populates the frames itself.
Zero extra API calls — the fetch is shared/memoized with extract_media. A
recapture now backfills the timings onto existing ugoira posts.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

backend/app/services/pixiv_downloader.py

@@ -102,11 +102,16 @@ class PixivDownloader(BaseNativeDownloader):
         validate: bool = True,
         rate_limit: float = 0.0,
         session: requests.Session | None = None,
+        ugoira_frames_fetcher: Callable[[dict], None] | None = None,
     ):
         super().__init__(
             images_root, cookies_path, platform="pixiv",
             validate=validate, rate_limit=rate_limit, session=session,
         )
+        # Injected by the ingester (client.fetch_ugoira_frames) so write_post_record
+        # can populate frame timings — which extract_media memoizes, but the core
+        # writes the post record FIRST. Mirrors Patreon's content_fetcher.
+        self._ugoira_frames_fetcher = ugoira_frames_fetcher
         if session is None:
             # i.pximg.net 403s any GET without the app Referer; mirror the
             # client's full app-header profile (gallery-dl serves media off
@@ -248,7 +253,16 @@ class PixivDownloader(BaseNativeDownloader):
                 "account": user.get("account"),
                 "name": user.get("name"),
             }
-        # Memoized by extract_media's ugoira branch; the zip has no timings.
+        # Ugoira frame timings. extract_media memoizes these, but the core writes
+        # the post record BEFORE extracting media, so fetch them here (shared +
+        # idempotent via the client's memoization) so the record actually keeps
+        # them — the zip carries no timings.
+        if (
+            work.get("type") == "ugoira"
+            and not work.get("_ugoira_frames")
+            and self._ugoira_frames_fetcher is not None
+        ):
+            self._ugoira_frames_fetcher(post)
         frames = work.get("_ugoira_frames")
         if frames:
             data["ugoira_frames"] = frames