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/ingest_core.py

@@ -143,6 +143,11 @@ class Ingester:
         # media, so phase 3 can backfill the ImageRecord's source_filehash WITHOUT
         # re-downloading or unlinking the file. Empty outside recapture mode.
         relink: list[tuple[str, str]] = []
+        # #842: per-post body-capture diagnostics surfaced on the DownloadEvent so
+        # the operator can see IN THE UI how each post was handled — crucially the
+        # post_type + final body length, which explains an empty body (e.g. a poll
+        # whose body the API never returns) without digging through worker logs.
+        post_diag: list[dict] = []
         downloaded = 0
         errors = 0
         quarantined = 0
@@ -180,6 +185,7 @@ class Ingester:
                 written_paths=written,
                 post_record_paths=list(post_records),
                 relink_source_paths=list(relink),
+                post_diagnostics=list(post_diag),
                 stdout="\n".join(log_lines),
                 stderr="",
                 return_code=return_code,
@@ -264,6 +270,19 @@ class Ingester:
                             if rec_path is not None:
                                 post_records.append(str(rec_path))
                                 self._mark_seen(source_id, [(pkey, ppid)])
+                            # Read the FINAL body (write_post_record memoized any
+                            # detail-fetched content onto the post) + post_type
+                            # (already in the feed fieldset) for the per-post UI
+                            # diagnostic. A body_chars of 0 with the post_type is
+                            # the operator's "why is this one empty" answer.
+                            pattrs = post.get("attributes") or {}
+                            pbody = pattrs.get("content")
+                            post_diag.append({
+                                "post_id": ppid,
+                                "title": pattrs.get("title") or None,
+                                "post_type": pattrs.get("post_type") or None,
+                                "body_chars": len(pbody) if isinstance(pbody, str) else 0,
+                            })
 
                 media = self.client.extract_media(post, included)
                 if not media: