refactor(ingest): per-post handling into run stdout via a downloader outcome (#842)

Two corrections from operator review:
1. Reuse the existing 'Raw stdout' panel instead of a bespoke structured UI
   section — the native ingester now writes a per-post line into the run stdout
   (parity with gallery-dl's per-file stdout), so the per-post handling shows in
   the panel the operator already uses.
2. DRY: stop re-reading post['attributes'] inline in ingest_core. write_post_record
   now returns a PostRecordOutcome (path, post_type, title, body_chars) — mirroring
   the download_post -> MediaOutcome contract — and the downloader owns the read;
   ingest_core only formats the outcome into the log line.

Reverts the post_diagnostics metadata field + DownloadDetailModal 'Post capture'
section added earlier. Per-post line: 'post <id> [<post_type>] body: N chars' (+
' — EMPTY' when 0), so an empty body is self-explanatory by post_type.

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

backend/app/services/download_service.py

@@ -477,10 +477,6 @@ 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,