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>

tests/test_download_service.py

@@ -60,10 +60,11 @@ def _make_fake_dl_result(
     *, success=True, written_paths=None, quarantined_paths=None,
     files_downloaded=0, error_type=None, error_message=None,
     stdout="", stderr="", cursor=None, run_stats=None, posts_processed=0,
-    relink_source_paths=None,
+    relink_source_paths=None, post_diagnostics=None,
 ):
     return SimpleNamespace(
         relink_source_paths=relink_source_paths or [],
+        post_diagnostics=post_diagnostics or [],
         success=success,
         url="https://patreon.com/alice",
         artist_slug="alice",
@@ -820,6 +821,28 @@ async def test_recapture_mode_selected_and_flag_cleared_on_complete(
     assert "_backfill_recapture" not in co
 
 
+@pytest.mark.asyncio
+async def test_post_diagnostics_written_to_event_metadata(
+    db, db_sync, tmp_path, seed_artist_and_source,
+):
+    """#842: per-post body diagnostics land on the DownloadEvent.metadata_ so the
+    UI can show how each post was handled (post_type + body length)."""
+    _artist, source = seed_artist_and_source
+    diag = [
+        {"post_id": "p1", "title": "Has body", "post_type": "image_file", "body_chars": 42},
+        {"post_id": "p2", "title": "Empty poll", "post_type": "poll", "body_chars": 0},
+    ]
+    svc, _ = _backfill_svc(db, db_sync, tmp_path, _make_fake_dl_result(
+        success=True, post_diagnostics=diag,
+    ))
+    await svc.download_source(source.id)
+
+    ev = (await db.execute(
+        select(DownloadEvent).order_by(DownloadEvent.id.desc()).limit(1)
+    )).scalar_one()
+    assert ev.metadata_["post_diagnostics"] == diag
+
+
 @pytest.mark.asyncio
 async def test_partial_error_type_maps_to_ok_status(
     db, db_sync, tmp_path, seed_artist_and_source,