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>

frontend/src/components/downloads/DownloadDetailModal.vue

@@ -28,6 +28,54 @@
           <div><span class="fc-dl-grid__label">Errors</span><span>{{ summary.errors ?? 0 }}</span></div>
         </div>
 
+        <template v-if="postDiag.length">
+          <h3 class="text-subtitle-2 mt-4">Post capture</h3>
+          <div class="fc-dl-grid">
+            <div><span class="fc-dl-grid__label">Posts</span><span>{{ postDiag.length }}</span></div>
+            <div><span class="fc-dl-grid__label">With body</span><span>{{ postWithBody }}</span></div>
+            <div><span class="fc-dl-grid__label">Empty body</span><span>{{ postEmpty.length }}</span></div>
+          </div>
+          <v-expansion-panels class="mt-2">
+            <v-expansion-panel v-if="postEmpty.length">
+              <v-expansion-panel-title>
+                <span>Empty-body posts ({{ postEmpty.length }})</span>
+              </v-expansion-panel-title>
+              <v-expansion-panel-text>
+                <table class="fc-dl-table">
+                  <thead><tr><th>Post</th><th>Type</th><th>Chars</th></tr></thead>
+                  <tbody>
+                    <tr v-for="p in postEmpty" :key="p.post_id" class="fc-dl-empty">
+                      <td>{{ p.title || `#${p.post_id}` }}</td>
+                      <td><code>{{ p.post_type || '—' }}</code></td>
+                      <td>{{ p.body_chars }}</td>
+                    </tr>
+                  </tbody>
+                </table>
+              </v-expansion-panel-text>
+            </v-expansion-panel>
+            <v-expansion-panel>
+              <v-expansion-panel-title>
+                <span>All posts ({{ postDiag.length }})</span>
+              </v-expansion-panel-title>
+              <v-expansion-panel-text>
+                <table class="fc-dl-table">
+                  <thead><tr><th>Post</th><th>Type</th><th>Chars</th></tr></thead>
+                  <tbody>
+                    <tr
+                      v-for="p in postDiag" :key="p.post_id"
+                      :class="{ 'fc-dl-empty': !p.body_chars }"
+                    >
+                      <td>{{ p.title || `#${p.post_id}` }}</td>
+                      <td><code>{{ p.post_type || '—' }}</code></td>
+                      <td>{{ p.body_chars }}</td>
+                    </tr>
+                  </tbody>
+                </table>
+              </v-expansion-panel-text>
+            </v-expansion-panel>
+          </v-expansion-panels>
+        </template>
+
         <template v-if="quarantinedPaths.length">
           <h3 class="text-subtitle-2 mt-4">Quarantined paths</h3>
           <ul class="fc-dl-quar">
@@ -115,18 +163,31 @@ const summary = computed(() => props.event?.metadata?.import_summary || {})
 const quarantinedPaths = computed(() => props.event?.metadata?.quarantined_paths || [])
 const errorsWarnings = computed(() => props.event?.metadata?.stderr_errors_warnings || '')
 
+// #842: per-post body-capture diagnostics — how each post was handled, with
+// post_type so an empty body is self-explanatory in the UI.
+const postDiag = computed(() => props.event?.metadata?.post_diagnostics || [])
+const postEmpty = computed(() => postDiag.value.filter((p) => !p.body_chars))
+const postWithBody = computed(() => postDiag.value.length - postEmpty.value.length)
+
 // One combined block for "research the issue elsewhere" — header line +
 // error + full stdout/stderr. Built lazily from the current event.
 const allDiagnostics = computed(() => {
   const e = props.event
   if (!e) return ''
   const md = e.metadata || {}
+  const emptyPosts = postEmpty.value
+    .map((p) => `  • ${p.title || '#' + p.post_id} [${p.post_type || '—'}] ${p.body_chars} chars`)
+    .join('\n')
   return [
     `event #${e.id} · ${e.platform || '—'} · ${e.artist_name || '—'}`,
     `status: ${e.status}`,
     `started: ${e.started_at}  finished: ${e.finished_at || '(running)'}`,
     e.error ? `\n--- error ---\n${e.error}` : '',
     errorsWarnings.value ? `\n--- errors & warnings ---\n${errorsWarnings.value}` : '',
+    postDiag.value.length
+      ? `\n--- post capture (${postWithBody.value}/${postDiag.value.length} with body) ---`
+      + (emptyPosts ? `\nempty-body posts:\n${emptyPosts}` : '')
+      : '',
     `\n--- stdout ---\n${md.stdout || '(empty)'}`,
     `\n--- stderr ---\n${md.stderr || '(empty)'}`,
   ].filter(Boolean).join('\n')
@@ -181,6 +242,25 @@ function onClose() {
   word-break: break-all;
 }
 .fc-dl-quar { padding-left: 1.5rem; font-size: 0.85rem; }
+.fc-dl-table {
+  width: 100%;
+  border-collapse: collapse;
+  font-size: 0.8rem;
+}
+.fc-dl-table th,
+.fc-dl-table td {
+  text-align: left;
+  padding: 0.2rem 0.5rem;
+  border-bottom: 1px solid rgb(var(--v-theme-on-surface-variant) / 0.14);
+}
+.fc-dl-table th:last-child,
+.fc-dl-table td:last-child {
+  text-align: right;
+  font-variant-numeric: tabular-nums;
+}
+.fc-dl-table .fc-dl-empty {
+  color: rgb(var(--v-theme-warning));
+}
 .fc-dl-blockhead {
   display: flex; align-items: center; justify-content: space-between;
   gap: 0.5rem;