Prepares publication of the Buzz push gateway chart 0.3.3, including the
optional migration Pod annotations already merged in #8105. This PR adds
release notes only; chart templates, values, and schema are unchanged
from the reviewed implementation.

Merging the `push-chart-release/0.3.3` branch triggers the normal
release workflow: create `push-chart-v0.3.3`, validate the chart, then
publish `oci://ghcr.io/block/buzz/charts/buzz-push-gateway:0.3.3`. Keep
this PR in draft until publication is explicitly authorized. It does not
deploy a gateway.

### Validation

Focused migration annotation renders, release contract checks, and local
chart packaging pass. The merged implementation passed chart CI and
Jude's automated review in #8105. The earlier repository-wide local
`just ci` run failed in unchanged ACP tests; it was not rerun for
release notes. The registry manifest digest can only be verified after
publication.

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