fix(posts): anchored view scroll inherits artist_id/platform filters

Operator-flagged 2026-06-01: clicking a Provenance post title from the
view modal opens the post in the posts feed scoped to that artist
(`/posts?post_id=N&artist_id=A`), but the older/newer infinite-scroll
loaded UNFILTERED global posts instead of staying in the artist's
stream. Root cause: the posts store's loadAround/loadOlder/loadNewer
sent only `{around, cursor, direction}` — never the `artist_id` /
`platform` filters. Backend `/api/posts` accepts them on every path
(post_feed_service.scroll filters via `Source.artist_id`); the
frontend just wasn't passing them.

Fix:
- `posts.js` `loadAround(postId, filters)` now takes the filter
  snapshot and stores it. `_aroundParams(extra)` mixes the active
  filters into around/cursor calls.
- `PostsView.vue` `loadAroundAndAnchor` passes `artist_id` +
  `platform` from the route query.

frontend/src/views/PostsView.vue

@@ -161,7 +161,15 @@ function setupAroundObservers() {
 }
 async function loadAroundAndAnchor() {
   teardownFeed()
-  await store.loadAround(postIdFilter.value)
+  // Pass artist_id + platform through so the anchored view stays
+  // scoped — the older/newer infinite scrolls then read these filters
+  // back via the store's _aroundParams (operator-flagged 2026-06-01:
+  // post-title click from the modal landed scoped but the scroll then
+  // pulled unfiltered global posts).
+  await store.loadAround(postIdFilter.value, {
+    artist_id: artistFilter.value,
+    platform: platformFilter.value,
+  })
   await nextTick()
   const el = document.getElementById(`fc-post-${store.anchorId}`)
   if (el) el.scrollIntoView({ block: 'center' })