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/stores/posts.js

@@ -59,14 +59,37 @@ export const usePostsStore = defineStore('posts', () => {
     return await api.get(`/api/posts/${id}`)
   }
 
+  // Filter overlay for the around/older/newer (in-context anchored)
+  // path. Keep this distinct from `filters.value` (the down-only feed)
+  // so a normal-feed filter change doesn't leak into an active anchored
+  // view (or vice versa). Caller of loadAround passes the snapshot; the
+  // subsequent loadOlder/loadNewer use it verbatim.
+  function _aroundParams(extra) {
+    const p = { ...extra }
+    if (filters.value.artist_id != null) p.artist_id = filters.value.artist_id
+    if (filters.value.platform) p.platform = filters.value.platform
+    return p
+  }
+
   // Load a window centered on `postId`: newer posts above, the post, older
   // posts below. Sets both directional cursors for subsequent scrolling.
-  async function loadAround(postId) {
+  // Accepts the same filter shape as loadInitial so the anchored view
+  // stays artist/platform-scoped (operator-flagged 2026-06-01: clicking a
+  // post title from the modal's Provenance card opens the post in the
+  // posts feed; without this the older/newer scroll loaded unfiltered
+  // global posts instead of staying in the artist's stream).
+  async function loadAround(postId, newFilters) {
+    filters.value = {
+      artist_id: newFilters?.artist_id ?? null,
+      platform: newFilters?.platform ?? null,
+    }
     loading.value = true
     error.value = null
     anchorId.value = null
     try {
-      const body = await api.get('/api/posts', { params: { around: postId } })
+      const body = await api.get('/api/posts', {
+        params: _aroundParams({ around: postId }),
+      })
       items.value = body.items
       cursorOlder.value = body.cursor_older
       cursorNewer.value = body.cursor_newer
@@ -85,7 +108,9 @@ export const usePostsStore = defineStore('posts', () => {
     loading.value = true
     try {
       const body = await api.get('/api/posts', {
-        params: { cursor: cursorOlder.value, direction: 'older' },
+        params: _aroundParams({
+          cursor: cursorOlder.value, direction: 'older',
+        }),
       })
       items.value.push(...body.items)
       cursorOlder.value = body.next_cursor
@@ -102,7 +127,9 @@ export const usePostsStore = defineStore('posts', () => {
     loading.value = true
     try {
       const body = await api.get('/api/posts', {
-        params: { cursor: cursorNewer.value, direction: 'newer' },
+        params: _aroundParams({
+          cursor: cursorNewer.value, direction: 'newer',
+        }),
       })
       items.value.unshift(...body.items)
       cursorNewer.value = body.next_cursor

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' })