feat: cache remote account stats for embedded status accounts

Phanpy never calls /accounts/:id for timeline authors — it uses the
embedded account object from the status response. These had 0 counts
because the timeline author data doesn't include follower stats.

Fix: in-memory LRU cache (500 entries, 1h TTL) stores account stats
from remote resolutions. serializeAccount() reads from cache when
the actor has 0 counts, enriching embedded accounts with real data.

Cache is populated by resolveRemoteAccount() (lookup, search, and
/accounts/:id calls). Once a profile has been viewed once, all
subsequent status embeds for that author show real counts.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@rmdes/indiekit-endpoint-activitypub",
-  "version": "3.6.6",
+  "version": "3.6.7",
   "description": "ActivityPub federation endpoint for Indiekit via Fedify. Adds full fediverse support: actor, inbox, outbox, followers, following, syndication, and Mastodon migration.",
   "keywords": [
     "indiekit",