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/gallery_dl.py

@@ -158,11 +158,6 @@ class DownloadResult:
     # backfilled (inline-image localization) WITHOUT re-download or unlink. Empty
     # on the gallery-dl path and outside recapture.
     relink_source_paths: list[tuple[str, str]] = field(default_factory=list)
-    # Native ingester (#842): per-post body-capture diagnostics
-    # ({post_id, title, post_type, body_chars}) surfaced on the DownloadEvent so
-    # the operator can see in the UI how each post was handled. Empty on the
-    # gallery-dl path and for posts whose body wasn't (re)captured this run.
-    post_diagnostics: list[dict] = field(default_factory=list)
     stdout: str = ""
     stderr: str = ""
     return_code: int = 0