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/download_service.py

@@ -406,7 +406,9 @@ class DownloadService:
         # their post-body inline <img src=CDN> remaps to the local copy. A
         # SEPARATE non-deleting channel (NOT the import list — that would unlink
         # the duplicate file). Empty outside recapture mode.
-        for rel_str, rel_url in getattr(dl_result, "relink_source_paths", None) or []:
+        relink_pairs = getattr(dl_result, "relink_source_paths", None) or []
+        relinked = 0
+        for rel_str, rel_url in relink_pairs:
             rel_path = Path(rel_str)
             if not rel_path.exists():  # noqa: ASYNC240
                 continue
@@ -414,7 +416,16 @@ class DownloadService:
             def _relink(p=rel_path, u=rel_url):
                 return self.importer.relink_source_filehash(p, u, artist=artist)
 
-            await loop.run_in_executor(None, _relink)
+            if await loop.run_in_executor(None, _relink):
+                relinked += 1
+        if relink_pairs:
+            # recapture diagnostic: how many on-disk images got their
+            # source_filehash backfilled (inline-image localization). < total is
+            # normal — files already carrying a filehash are skipped (NULL-only).
+            log.info(
+                "recap: relinked source_filehash on %d/%d on-disk image(s)",
+                relinked, len(relink_pairs),
+            )
 
         # Kick the off-platform file-host downloader for any links this run
         # recorded (mega/gdrive/…). Global + idempotent (only claims pending/