feat(gallery): reveal tiles on image load + single initial fetch

The 5×10 metadata batching only staggered the cheap layer (JSON);
thumbnails load as independent <img> requests and clustered, so tiles
"popped in together" after a wait. Two changes:

- GalleryItem reveals each tile when ITS OWN thumbnail fires @load (with
  an onMounted complete-check for cached thumbs), playing a showcase-style
  flip-up entrance. Tiles now cascade in natural load order instead of all
  at once. Honors prefers-reduced-motion.
- gallery store does ONE initial fetch (limit=50) instead of 10 serial
  /scroll round-trips. Fewer RTTs, faster first paint; the reveal-on-load
  is what makes appearance progressive now. Infinite scroll pulls 25/trigger.

Tests: GalleryItem gains is-loaded only after @load; loadInitial issues
exactly one scroll request at the initial limit.

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

frontend/src/components/gallery/GalleryItem.vue

@@ -9,11 +9,16 @@
   >
     <div class="fc-gallery-item__media">
       <img
-        v-if="!isVideo" :src="image.thumbnail_url" :alt="`Image ${image.id}`"
-        loading="lazy" @error="onThumbError"
+        v-if="!isVideo" ref="imgEl" :src="image.thumbnail_url"
+        :alt="`Image ${image.id}`" loading="lazy"
+        :class="{ 'is-loaded': loaded }"
+        @load="loaded = true" @error="onThumbError"
       >
       <div v-else class="fc-gallery-item__video-thumb">
-        <img :src="image.thumbnail_url" :alt="`Video ${image.id}`" loading="lazy">
+        <img
+          ref="imgEl" :src="image.thumbnail_url" :alt="`Video ${image.id}`"
+          loading="lazy" :class="{ 'is-loaded': loaded }" @load="loaded = true"
+        >
         <v-icon class="fc-gallery-item__video-badge" icon="mdi-play-circle" />
       </div>
       <div v-if="thumbError" class="fc-gallery-item__placeholder">
@@ -38,7 +43,7 @@
 </template>
 
 <script setup>
-import { computed, ref } from 'vue'
+import { computed, onMounted, ref } from 'vue'
 import { useGallerySelectionStore } from '../../stores/gallerySelection.js'
 
 const props = defineProps({ image: { type: Object, required: true } })
@@ -46,6 +51,18 @@ const emit = defineEmits(['open'])
 
 const sel = useGallerySelectionStore()
 const thumbError = ref(false)
+
+// Reveal each tile when ITS OWN thumbnail finishes loading (`@load`), not
+// when its metadata batch lands — so tiles fade/flip in individually in
+// load order instead of popping in together (operator-flagged 2026-06-04).
+const loaded = ref(false)
+const imgEl = ref(null)
+onMounted(() => {
+  // A cached thumbnail can already be complete before @load binds; reflect
+  // that so the tile reveals instead of sitting invisible at opacity 0.
+  const el = imgEl.value
+  if (el && el.complete && el.naturalWidth > 0) loaded.value = true
+})
 const isVideo = computed(
   () => props.image.mime && props.image.mime.startsWith('video/')
 )
@@ -82,6 +99,30 @@ function onThumbError() { thumbError.value = true }
 }
 .fc-gallery-item__media img {
   width: 100%; height: 100%; object-fit: cover; display: block;
+  opacity: 0;
+}
+/* Entrance: each thumbnail flips up out of a slight backward tilt and
+   settles into place, played WHEN ITS OWN image finishes loading (the
+   `is-loaded` class) rather than on a batch/timer — so tiles cascade in
+   natural load order instead of popping in together. Mirrors the showcase
+   MasonryGrid entrance styling (operator-flagged 2026-06-04). */
+.fc-gallery-item__media img.is-loaded {
+  animation: fc-gallery-item-in 0.5s cubic-bezier(0.34, 1.45, 0.64, 1) both;
+}
+@keyframes fc-gallery-item-in {
+  0% {
+    opacity: 0;
+    transform: perspective(1000px) rotateX(-22deg) translateY(20px) scale(0.96);
+  }
+  55% { opacity: 1; }
+  100% {
+    opacity: 1;
+    transform: perspective(1000px) rotateX(0) translateY(0) scale(1);
+  }
+}
+@media (prefers-reduced-motion: reduce) {
+  .fc-gallery-item__media img { opacity: 1; }
+  .fc-gallery-item__media img.is-loaded { animation: none; }
 }
 .fc-gallery-item__video-thumb { position: relative; height: 100%; }
 .fc-gallery-item__video-badge {