feat(ingest): per-post body-capture + recapture diagnostics logging

Operator-flagged: a recapture 'caught nothing' for a post and there were no
logs explaining why. Three silent spots now log, so a recapture's per-post
outcome is diagnosable (retention bounds the volume):

- patreon_client.fetch_post_detail_content: the 200-OK-but-null-content branch
  was silent — now logs 'fetched N chars' on success AND 'empty/null content
  (tier-gated or no text)' on the empty case (the most common silent miss).
- patreon_downloader.write_post_record: logs each post's FINAL body outcome
  (captured N chars / NO body) read off the memoized attrs after detail-fetch.
- ingest_core summary: appends post-record + relinked counts to the run summary
  (surfaces on the event stdout the operator already reads).
- download_service phase3: logs how many on-disk images got source_filehash
  relinked (N/total) per recapture.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/services/patreon_downloader.py

@@ -574,8 +574,20 @@ class PatreonDownloader:
         leading underscore keeps it from colliding with a media sidecar
         (`<NN>_<stem>.json`) and from being resolved as some media file's sidecar
         by find_sidecar. Returns the path, or None when the post has no id."""
-        if not str(post.get("id") or ""):
+        pid = str(post.get("id") or "")
+        if not pid:
             return None
         post_dir = self.images_root / artist_slug / "patreon" / _post_dir_name(post)
         post_dir.mkdir(parents=True, exist_ok=True)
-        return self._write_sidecar_data(post, post_dir / "_post.json")
+        path = self._write_sidecar_data(post, post_dir / "_post.json")
+        # Per-post body-capture diagnostic: _write_sidecar_data has, by now,
+        # memoized any detail-fetched body onto post["attributes"]["content"], so
+        # this reflects the FINAL body (feed or detail). Logs the success (N chars)
+        # AND the skip (empty) so a recapture's per-post outcome is visible.
+        body = (post.get("attributes") or {}).get("content")
+        n = len(body) if isinstance(body, str) else 0
+        if n:
+            log.info("post-record: post %s body captured (%d chars)", pid, n)
+        else:
+            log.info("post-record: post %s has NO body (feed + detail both empty)", pid)
+        return path