feat(helm): make the PVC access modes configurable (#3061)

persistence.data.accessModes and persistence.uploads.accessModes default to
ReadWriteOnce, so existing installs render the same claims. ReadWriteMany lets a
backup pod mount the volumes; values.yaml and the docs warn that SQLite allows
only one writer of travel.db. CI renders the override too.

.github/workflows/test.yml

@@ -1276,6 +1276,10 @@ jobs:
             --set httpRoute.enabled=true \
             --set 'httpRoute.parentRefs[0].name=gateway' \
             > "$out/http-route.yaml"
+          helm template trek charts/trek \
+            --set 'persistence.data.accessModes[0]=ReadWriteMany' \
+            --set 'persistence.uploads.accessModes[0]=ReadWriteMany' \
+            > "$out/access-modes.yaml"
           helm template trek charts/trek \
             --set podSecurityContext.runAsUser=1000 \
             --set podSecurityContext.runAsGroup=1000 \