feat(series): pending staging for add-from-post (#789 Phase 2)

Add-from-post no longer appends straight into the run — it STAGES the post's
pages as pending (per-page status; page_number NULL), grouped by source post,
so the operator drops junk (text-free alts, bumpers) and places the keepers
into the sequence with clean series-global numbering.

- migration 0048: series_page.status ('placed' default | 'pending') + nullable
  page_number.
- series_service: placed/pending split everywhere (list_pages returns the
  placed run + a `pending` section grouped by source post; reorder/cover/
  list_series operate on placed only); add_post stages pending; new
  place_pending(image_ids, before_image_id=None) flips pending→placed spliced
  before a page (or appended) and renumbers; junk removal reuses remove_images.
- api/tags: /add-post now returns staged count; new POST /series/<id>/pending/
  place.
- frontend: PostSeriesMenu navigates to the series after staging; seriesManage
  store surfaces `pending` + placePending; SeriesManageView gains a pending
  tray (per-post groups, place-all / place-one / drop-junk).
- tests: pending staging, place (append + insert-before), ignore-already-
  placed, drop-junk, route guard; updated add_post + match-accept expectations.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/views/SeriesManageView.vue

@@ -32,6 +32,51 @@
       be dragged into place.
     </p>
 
+    <!-- Pending tray: pages staged from a post, grouped by source post, for the
+         operator to drop junk and place into the run. -->
+    <div v-if="store.pending.length" class="fc-pending">
+      <div class="fc-pending__head">
+        <v-icon size="small">mdi-tray-arrow-down</v-icon>
+        <span>Pending — sort into the series</span>
+      </div>
+      <p class="fc-pending__hint">
+        New pages from a post land here. Drop the ones that don't belong
+        (text-free alternates, bumpers), then place the rest into the run.
+      </p>
+      <section v-for="(grp, gi) in store.pending" :key="gi" class="fc-pgroup">
+        <header class="fc-pgroup__head">
+          <span class="fc-pgroup__title" :title="grp.post?.title">
+            <v-icon size="x-small">mdi-link-variant</v-icon>
+            {{ grp.post?.title || 'Unknown post' }}
+          </span>
+          <span class="fc-pgroup__count">{{ grp.pages.length }} pending</span>
+          <v-spacer />
+          <v-btn
+            size="small" variant="tonal" color="accent"
+            prepend-icon="mdi-playlist-plus"
+            @click="store.placePending(grp.pages.map(p => p.image_id))"
+          >Place all into series</v-btn>
+        </header>
+        <div class="fc-pgroup__pages">
+          <div v-for="p in grp.pages" :key="p.image_id" class="fc-ppage">
+            <img :src="p.thumbnail_url" alt="" loading="lazy" />
+            <div class="fc-ppage__actions">
+              <v-btn
+                size="x-small" variant="flat" icon="mdi-playlist-plus"
+                title="Place this page into the series"
+                @click="store.placePending([p.image_id])"
+              />
+              <v-btn
+                size="x-small" variant="flat" icon="mdi-close"
+                title="Drop — doesn't belong in the series"
+                @click="store.remove(p.image_id)"
+              />
+            </div>
+          </div>
+        </div>
+      </section>
+    </div>
+
     <!-- One continuous page run; chapter dividers render inline before their
          anchor page. -->
     <div v-if="store.pages.length" class="fc-run">
@@ -337,6 +382,57 @@ onMounted(async () => {
   max-width: 1100px;
 }
 
+/* Pending tray */
+.fc-pending {
+  max-width: 1100px; margin-bottom: 18px;
+  border: 1px solid rgb(var(--v-theme-accent), 0.4);
+  border-radius: 10px; padding: 10px 14px;
+  background: rgb(var(--v-theme-accent), 0.06);
+}
+.fc-pending__head {
+  display: flex; align-items: center; gap: 8px;
+  font-family: 'Fraunces', Georgia, serif; font-size: 16px;
+  color: rgb(var(--v-theme-accent));
+}
+.fc-pending__hint {
+  font-size: 12px; color: rgb(var(--v-theme-on-surface-variant));
+  margin: 4px 0 10px;
+}
+.fc-pgroup { margin-bottom: 12px; }
+.fc-pgroup__head {
+  display: flex; align-items: center; gap: 10px; margin-bottom: 6px;
+}
+.fc-pgroup__title {
+  display: inline-flex; align-items: center; gap: 4px; max-width: 360px;
+  overflow: hidden; text-overflow: ellipsis; white-space: nowrap;
+  font-size: 13px; font-weight: 600;
+}
+.fc-pgroup__count {
+  font-size: 12px; color: rgb(var(--v-theme-on-surface-variant));
+}
+.fc-pgroup__pages {
+  display: grid; gap: 8px;
+  grid-template-columns: repeat(auto-fill, minmax(110px, 1fr));
+}
+.fc-ppage {
+  position: relative; border-radius: 6px; overflow: hidden;
+  background: rgb(var(--v-theme-surface-light));
+  border: 1px dashed rgb(var(--v-theme-accent), 0.5);
+}
+.fc-ppage img {
+  width: 100%; aspect-ratio: 3 / 4; object-fit: contain; display: block;
+  background: rgb(var(--v-theme-background));
+}
+.fc-ppage__actions {
+  position: absolute; top: 4px; right: 4px; display: flex; gap: 2px;
+  opacity: 0; transition: opacity 0.12s;
+}
+.fc-ppage:hover .fc-ppage__actions,
+.fc-ppage:focus-within .fc-ppage__actions { opacity: 1; }
+.fc-ppage__actions :deep(.v-btn) {
+  background: rgba(0, 0, 0, 0.55); color: #fff;
+}
+
 /* Picker slide-over */
 .fc-picker__head {
   position: sticky; top: 0; z-index: 1; padding: 12px 14px;