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>

tests/test_patreon_ingester.py

@@ -637,6 +637,9 @@ async def test_backfill_skips_already_captured_post_but_recapture_forces_it(
     assert len(res_recap.relink_source_paths) == 1
     rel_path, rel_url = res_recap.relink_source_paths[0]
     assert rel_url == m1.url
+    # Diagnostic summary surfaces the post-record + relink counts (operator reads
+    # this off the event stdout to see what a recapture actually did).
+    assert "1 post-record(s), 1 relinked" in res_recap.stdout
 
 
 @pytest.mark.asyncio