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/gallery_dl.py

@@ -158,6 +158,11 @@ 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