Publish push gateway chart 0.3.4 through the normal chart release
workflow. The chart version and implementation were merged in
[#8113](https://github.com/block/buzz/pull/8113); this release PR
records the release notes and triggers the immutable chart tag and OCI
publisher.

Publication is authorized. The pending security review of #8113 is a
blocker for pinning the chart in the later infrastructure PR, not for
this publication. No gateways or relays are activated.

### Validation

The merged implementation passed the complete chart render suite, chart
CI, and all 14 local wrapper tests. This release changes only the
changelog; chart contents are unchanged from the validated
implementation. Published OCI contents and digest will be verified after
publication.

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Signed-off-by: Tom Brow <tomb@block.xyz>