feat(push): make gateway origin configurable (#7261)

Make the push gateway origin configurable across the relay, gateway,
mobile client, and deployment chart.

This is a fresh deployment. The gateway has only run on personal
development infrastructure, and push has no existing enabled users.
Legacy migration, configured-to-unconfigured mobile transitions, and
rolling upgrades from pre-launch gateway binaries are not supported.

Populated pre-launch databases are rejected with an informative error
before migrations run; their data is untouched. Mobile builds may omit
the gateway URL, leaving push unavailable. Once push is enabled,
subsequent builds must retain that configuration.

### Validation

- All applicable CI checks pass at
`5f546cfaf403f0386f146d213adfb4392f4ec29c`, including Rust,
mobile/Swift, desktop/integration, security, image builds, and Helm
checks. An unrelated agent cancellation test passed on retry.
- PostgreSQL CI passed 384 tests, including the production-entry-point
regression for refusing populated pre-launch databases before
migrations, fresh initialization, and subsequent initialized
deployments.
- Focused local validation passed all 10 gateway PostgreSQL tests under
the CI discovery filter and all 3 unconfigured-mobile tests.
Repository-wide PostgreSQL discovery validation and the complete
pre-push hooks passed.
- Physical-device APNs/App Attest behavior and deployment to a live
cluster were not verified in this work.

---------

Signed-off-by: Tom Brow <tomb@block.xyz>
Co-authored-by: Codex <noreply@openai.com>

deploy/compose/README.md

@@ -38,6 +38,10 @@ keypair.
   migrations.
 - The stack uses Postgres, Redis, MinIO, and a git data volume because
   those are real Buzz dependencies today. Minimal mode can simplify this later.
+- Mobile push remains off by default. To use the public gateway, keep the
+  template's explicit `BUZZ_PUSH_GATEWAY_DELIVERY_URL` and set
+  `BUZZ_PUSH_ENABLED=true`. To use another gateway, replace the exact HTTPS
+  `/v1/deliveries/apns` URL before enabling push.
 - The bundled Compose stack fixes the relay endpoint to `http://minio:9000` and
   `BUZZ_S3_ADDRESSING_STYLE=path`: Docker DNS resolves `minio`, not
   `<bucket>.minio`. It is not configurable for an external S3 provider through