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/ingest_core.py

@@ -402,7 +402,8 @@ class Ingester:
             f"{self._platform} ingest ({mode}): {downloaded} downloaded, "
             f"{skipped_count} skipped, {quarantined} quarantined, "
             f"{dead_lettered} dead-lettered, {errors} error(s), "
-            f"{posts_processed} post(s)"
+            f"{posts_processed} post(s), {len(post_records)} post-record(s), "
+            f"{len(relink)} relinked"
             + (", reached end" if reached_bottom else "")
             + (", time-boxed" if budget_hit else "")
         )