chore(deps): update activitypub fork to v3.6.8 (Mastodon Client API merge)

Pulls the merged upstream feat/mastodon-client-api branch into our fork
(svemagie/indiekit-endpoint-activitypub@f029c31). The fork now tracks
upstream v3.6.8 while retaining our custom additions.

What's new in the installed package:
- lib/mastodon/ — full Mastodon Client API compatibility layer (entities,
  middleware, routes, helpers, backfill-timeline, router)
- 13 additional locale files (es, fr, de, hi, id, it, nl, pt, sr, sv, zh, …)
- signatureTimeWindow and allowPrivateAddress built into federation-setup.js
  (patch-ap-allow-private-address now cleanly detects "already up to date")

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package-lock.json

@@ -2356,8 +2356,8 @@
       }
     },
     "node_modules/@rmdes/indiekit-endpoint-activitypub": {
-      "version": "2.15.4",
+      "version": "3.6.8",
-      "resolved": "git+ssh://git@github.com/svemagie/indiekit-endpoint-activitypub.git#842fc5af2ab946d6cc2f2abfbe4728da3edaf3da",
+      "resolved": "git+ssh://git@github.com/svemagie/indiekit-endpoint-activitypub.git#f029c3128e4f47a4213c01264b816d76c170095e",
       "license": "MIT",
       "dependencies": {
         "@fedify/debugger": "^2.0.0",
@@ -2373,6 +2373,7 @@
         "node": ">=22"
       },
       "peerDependencies": {
+        "@indiekit/endpoint-micropub": "^1.0.0-beta.25",
         "@indiekit/error": "^1.0.0-beta.25",
         "@indiekit/frontend": "^1.0.0-beta.25"
       }