feat(downloads): per-post body-capture diagnostics in the event UI (#842)

Operator can't (and shouldn't have to) hunt worker logs to see why a recapture
left a post body empty. Surface per-post handling ON THE EVENT, in the UI.

The feed already requests post_type (in _FIELDS_POST), so ingest_core builds a
per-post diagnostic {post_id, title, post_type, body_chars} with zero extra
fetching — a 0-char body next to its post_type explains an empty post at a
glance (e.g. polls/embeds whose body the API never returns).

- ingest_core: accumulate post_diagnostics; thread via DownloadResult
- download_service: write to DownloadEvent.metadata_['post_diagnostics']
- DownloadDetailModal: 'Post capture' section — totals + empty-body table
  (post_type + chars, flagged) + all-posts table; included in Copy-all
- tests: ingester diag (post_type + body_chars), download_service metadata

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

backend/app/services/download_service.py

@@ -477,6 +477,10 @@ class DownloadService:
             # recurring "post shows a zip but no images" report. Each entry is
             # {file, reason}; None when every archive extracted cleanly.
             "unextracted_archives": unextracted_archives or None,
+            # #842: per-post body-capture outcomes ({post_id, title, post_type,
+            # body_chars}) so the UI shows how each post was handled — a 0-char
+            # body next to its post_type explains an empty post at a glance.
+            "post_diagnostics": getattr(dl_result, "post_diagnostics", None) or None,
         }
         await self._update_source_health(
             source_id=ctx["source_id"], status=status, error_message=ev.error,