Publish push gateway chart 0.4.0 through the repository's protected
release lane. The chart implementation and version already landed in
[#8206](https://github.com/block/buzz/pull/8206); this PR adds release
notes without changing runtime behavior.

Merging `push-chart-release/0.4.0` invokes the release bot to create
`push-chart-v0.4.0` and publish the immutable chart to GHCR. Publication
unblocks [the wrapper
chart](https://github.com/squareup/cash-helm-charts/pull/5740) and
subsequent disabled-production preparation. This does not activate any
gateway.

### Validation

Release notes match the merged single-application configuration and
migration guards. Diff checks pass. The source PR's required checks and
PostgreSQL suite passed; chart validation will run again on this release
PR.

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