Cache the merged media catalog and trim card rendering

CURRENT.md

@@ -40,6 +40,14 @@ mobile draft at 390px.
   - [x] C11 preview caching: Discord-hosted image/GIF previews (WebP 320px, hash-addressed, 5 MiB
     cap, one attempt) run after every walk in both modes. Live run `...-20260916090722`:
     `previewed: 2`; thumbs served over `/api/media/<scrape>/<item>/thumb` (200).
+  - [x] C61 Media browsing performance. The grid's merge step rescanned every scrape database per
+    request (about 300 ms for a page, 250 ms for stats, both refetched on every scroll append).
+    `MediaService` now keeps a candidate cache keyed by a scrape revision plus the normalized
+    filter shape (paging and empties dropped): one scan per filter set, then ~5 ms per page, and
+    stats share the scan. The client skips stats when the filters are unchanged (60 s TTL), updates
+    only the pager instead of rebuilding the toolbar on page loads, and renders tiles with
+    `content-visibility`/`decoding=async`/low fetch priority. Measured: 302 ms first page, 5 ms
+    later pages; smoke test streamed 2,480 tiles with no console errors.
   - [x] C60 Incremental re-scrapes and archive cleanup. A new scrape of a channel that already
     has a completed walk now starts after that walk's newest message (the best completed scrape by
     message count), so it only covers the newer gap instead of re-fetching history; the previous