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_ingester.py

@@ -81,6 +81,10 @@ class PixivIngester(Ingester):
             if downloader is not None
             else PixivDownloader(
                 self.images_root, cookies_path, validate=validate, rate_limit=rate_limit,
+                # write_post_record runs before extract_media in the core, so it
+                # fetches ugoira frame timings via the SAME client (shared,
+                # memoized) — else the record's ugoira_frames stays empty.
+                ugoira_frames_fetcher=resolved_client.fetch_ugoira_frames,
             )
         )
         super().__init__(