feat(homelab): isolate PR runner and add Paperless

.gitea/runner/pr-config.yaml

@@ -3,6 +3,10 @@ runner:
   capacity: 1
   timeout: 5h
   labels:
-    - homelab-pr:host
+    - homelab-pr:docker://docker.gitea.com/runner-images:ubuntu-24.04-v26.09.01
 cache:
   enabled: false
+container:
+  privileged: false
+  valid_volumes: []
+  docker_host: "-"