Publish Helm chart to Artifact Hub

The chart is published to `oci://ghcr.io/cryptomator/charts/cryptomator-hub` (signed with cosign, SLSA provenance attached) but is not discoverable on [Artifact Hub](https://artifacthub.io/).

## Tasks

- [ ] Add the missing chart metadata to `chart/Chart.yaml`:
- top-level `maintainers`
- `artifacthub.io/license: AGPL-3.0-only`
- `artifacthub.io/images` listing `ghcr.io/cryptomator/hub`, `ghcr.io/cryptomator/keycloak` and `postgres` (enables image scanning and shows the images in the listing)
- `artifacthub.io/links` (docs, issue tracker, cryptomator.org/hub)
- `artifacthub.io/prerelease: "true"` for `-rc` builds (set by the `helm-chart.yml` workflow so rc versions don't show as "latest")
- optionally `artifacthub.io/changes` per release, if it can be generated from the release notes
- [ ] Register the OCI repository on Artifact Hub (repository kind *Helm charts*, URL `oci://ghcr.io/cryptomator/charts/cryptomator-hub`) under a Cryptomator organization and claim ownership (`artifacthub-repo.yml` in the OCI repo, or the OCI metadata annotation route documented at )
- [ ] Verify that the cosign signature and provenance attestation are detected ("Signed" / "Provenance" badges)
- [ ] Link the Artifact Hub listing from `chart/README.md`

## Notes

`catalog.cattle.io/*` annotations for Rancher are already in place; the Artifact Hub annotations complement them and are ignored by Helm and Rancher.

Start with chart/Chart.yaml and chart/README.md, then inspect the helm-chart.yml workflow and Artifact Hub's Helm chart repository documentation. Add the specified metadata, register and claim the OCI repository, and verify the listing shows the expected images, Signed and Provenance badges, prerelease handling, and README link.

Written by the indexing model from the issue text.