Migrate Joplin to K3s and add existing k8s manifests

- New k8s/joplin/ deployment for note.expertfab.de (Postgres + Server +
  Traefik ingress with cert-manager), replicas=2 to match cluster size
- coredns-custom.yaml: route note.expertfab.de internally to Traefik LB
- Commit previously-built k8s manifests (documenso, erpnext oauth2-proxy,
  paperless oauth2-proxy) that were running but not in git
- docs/access.md: add Joplin section and Documenso/Cloudflare entries

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

k8s/documenso/namespace.yaml

@@ -0,0 +1,19 @@
+---
+apiVersion: v1
+kind: Namespace
+metadata:
+  name: documenso
+---
+apiVersion: storage.k8s.io/v1
+kind: StorageClass
+metadata:
+  name: longhorn-documenso
+provisioner: driver.longhorn.io
+reclaimPolicy: Retain
+allowVolumeExpansion: true
+volumeBindingMode: Immediate
+parameters:
+  numberOfReplicas: "2"
+  staleReplicaTimeout: "30"
+  fromBackup: ""
+  fsType: "ext4"