Release v1.2.1

## Release Checklist

> [!IMPORTANT]
> This issue tracks the status of this release. The subgraph deployment is automated via the [release-please workflow](https://github.com/FilOzone/filecoin-pay-explorer/actions/workflows/release-please.yml). Static site changes are automatically deployed via [Vercel](https://github.com/organizations/FilOzone/settings/installations/85630569).
>
> For a breaking schema change, deploy a compatible Explorer before promoting the new subgraphs to `prod`.

### 1. Confirm the release

- [ ] The Release Please PR for `v1.2.1` was merged into `staging`
- [ ] The `v1.2.1` tag and GitHub Release were created
- [ ] The `Deploy subgraph to Goldsky (calibration)` job succeeded
- [ ] The `Deploy subgraph to Goldsky (mainnet)` job succeeded
- [ ] The `Tag as staging (calibration)` job succeeded
- [ ] The `Tag as staging (mainnet)` job succeeded

If either deploy or tag job failed, fix and rerun it or follow the [manual deployment steps](https://github.com/FilOzone/filecoin-pay-explorer/blob/main/packages/subgraph/README.md#manually-deploying-the-subgraph-to-goldsky).

### 2. Await subgraph indexing

Check indexing in the [Goldsky subgraphs dashboard](https://app.goldsky.com/project_cmb9tuo8r1xdw01ykb8uidk7h/dashboard/subgraphs).

- [ ] Received confirmation that `filecoin-pay-mainnet/1.2.1` has finished indexing
- [ ] Received confirmation that `filecoin-pay-calibration/1.2.1` has finished indexing

### 3. Verify the candidate

Set the following environment variables locally and smoke-test the Explorer:

```bash
NEXT_PUBLIC_SUBGRAPH_URL_MAINNET=https://api.goldsky.com/api/public/project_cmb9tuo8r1xdw01ykb8uidk7h/subgraphs/filecoin-pay-mainnet/1.2.1/gn
NEXT_PUBLIC_SUBGRAPH_URL_CALIBRATION=https://api.goldsky.com/api/public/project_cmb9tuo8r1xdw01ykb8uidk7h/subgraphs/filecoin-pay-calibration/1.2.1/gn
```

- [ ] Explorer loads without errors on mainnet
- [ ] Explorer loads without errors on calibration
- [ ] Changes included in this release were smoke-tested

### 4. Promote to production

> [!NOTE]
> Merging the promotion PR triggers the [tag-subgraph-prod workflow](https://github.com/FilOzone/filecoin-pay-explorer/actions/workflows/tag-subgraph-prod.yml), which applies the `prod` tag on Goldsky and switches [pay.filecoin.cloud](https://pay.filecoin.cloud) to the new version. Do this only after indexing is confirmed and staging has been verified.

- [ ] Merge the `staging → main` promotion PR
- [ ] The `tag-subgraph-prod` workflow succeeded
- [ ] `filecoin-pay-mainnet/1.2.1` is tagged as `prod`
- [ ] `filecoin-pay-calibration/1.2.1` is tagged as `prod`

### 5. Verify production

- [ ] [pay.filecoin.cloud](https://pay.filecoin.cloud) loads and functions correctly on mainnet after the tag switch
- [ ] [pay.filecoin.cloud](https://pay.filecoin.cloud) loads and functions correctly on calibration after the tag switch

### 6. Wrap up

- [ ] The release was announced in #fil-foc
- [ ] Any improvements needed in the release process were documented
- [ ] Close this issue

Start with the release-please workflow and the subgraph README's manual deployment steps. Confirm the v1.2.1 PR, tag, GitHub Release, Goldsky calibration and mainnet deployment, and staging tags, then check indexing in the Goldsky dashboard. Smoke-test both Explorer environments with the listed subgraph URLs and complete production promotion and verification only after staging is confirmed.

Written by the indexing model from the issue text.