feat(ansible): migrate OpenWrt package management from opkg to apk

OpenWrt 25.12+ uses apk instead of opkg. The community.openwrt.apk
module is only available in the unreleased 1.4.0 (git main), so
requirements.yml now installs the collection from git.

- requirements.yml: install community.openwrt from git main branch
  (comment explains why — apk module not yet in a Galaxy release)
- packages.yml: switch from community.openwrt.opkg to apk, use
  join filter to pass all packages at once instead of looping

ansible/roles/openwrt/tasks/packages.yml

@@ -1,7 +1,7 @@
 ---
 - name: Install packages
-  community.openwrt.opkg:
-    name: "{{ item }}"
+  community.openwrt.apk:
+    name: "{{ openwrt_packages | join(',') }}"
     state: present
     update_cache: true
-  loop: "{{ openwrt_packages }}"
+  when: openwrt_packages | length > 0