ci(goreleaser): mirror release artifacts to Cloudflare R2 (#1063)

Ports the Cloudflare R2 release mirror from [gitea.com/gitea/runner](https://gitea.com/gitea/runner) to `tea`, so release artifacts land in R2 alongside S3 for the duration of the migration away from S3.

Two commits, meant to be reviewed in order.

## 1. `ci(goreleaser): migrate release config to goreleaser v2`

A purely mechanical migration, no behaviour change intended:

- add `version: 2`
- `blobs.folder` -> `blobs.directory`
- `snapshot.name_template` -> `snapshot.version_template`
- `nightly.name_template` -> `nightly.version_template`
- `version: "~> v1"` -> `"~> v2"` in both release workflows

This is a prefactor rather than scope creep. Under goreleaser v1 the custom-publisher pipe runs *4th*, before the `release` pipe; under v2 it runs *last*. That ordering difference matters for the change below: on v1 a failed R2 upload would abort the publish after every artifact had already gone to S3 but **before** the Gitea release was created, leaving a half-finished release. On v2 the R2 mirror runs after the release exists, which matches the behaviour the runner repo already has in production.

The pre-existing `archives.format` deprecation warning is deliberately left alone; it is orthogonal to this change and the runner repo has not addressed it either.

## 2. `ci(goreleaser): mirror release artifacts to Cloudflare R2`

- **`scripts/upload-r2.sh`** — uploads one local file to one R2 object key using curl's built-in AWS SigV4 signer (R2 is S3-API compatible). Credentials are fed through `curl --config -` so they never appear in `ps` output. Also provides a `--check-config` preflight mode. This file is byte-identical to the runner repo's copy.
- **`.goreleaser.yaml`** — a `publishers:` entry mirroring the existing S3 `blobs:` upload into R2. A second `blobs:` entry is not usable here: the blob pipe authenticates from the global `AWS_*` environment and has no per-entry credentials, whereas `publishers:` supports per-entry `env:`.
- **Both release workflows** — forward the R2 secrets, plus an early `check R2 configuration` step. Custom publishers run as the very last step of goreleaser's publish pipeline, so without a preflight a missing secret would only surface after the release had been created and every artifact already uploaded to S3.

### Deviation from the runner implementation

The publisher here also sets `signature: true` in addition to `checksum: true`. `tea` has a `signs:` block that GPG-signs the checksum file, and the S3 blob pipe uploads the resulting `checksums.txt.sig`; without `signature: true` the R2 mirror would carry the artifacts and their checksums but no signature to verify them against.

The object key prefix is `tea/{{ .Version }}/...`, matching the existing S3 `directory: "tea/{{.Version}}"`.

## Required repository secrets

This PR is inert until these are configured. The preflight step will fail the release loudly if they are missing:

- `R2_ENDPOINT` — e.g. `https://<account>.r2.cloudflarestorage.com`
- `R2_BUCKET`
- `R2_ACCESS_KEY_ID`
- `R2_SECRET_ACCESS_KEY`

## Verification

- `goreleaser check` against the migrated config (with the pro-only `nightly:` block temporarily stripped, since the check ran with the OSS v2 binary): *configuration is valid*, the only deprecation being the pre-existing `archives.format`.
- A real `goreleaser build --snapshot --clean --single-target` against the v2 config: succeeded, including the `xz` and `.goreleaser.checksum.sh` post-hooks.
- `scripts/upload-r2.sh`: clean under `sh -n` and `shellcheck`; all four `--check-config` cases exercised (all vars unset, one missing, all set, wrong argument count).
- The actual upload path was not exercised end to end, since that needs live R2 credentials.

Reviewed-on: https://gitea.com/gitea/tea/pulls/1063
Reviewed-by: Zettat123 <39446+zettat123@noreply.gitea.com>

.gitea/workflows/release-nightly.yml

@@ -12,6 +12,18 @@ jobs:
         with:
           fetch-depth: 0
       - run: git fetch --force --tags
+      # Custom publishers (the R2 mirror below) run as the very last
+      # step of goreleaser's publish pipeline, after the Gitea release
+      # has already been created and every artifact already uploaded
+      # to S3. Fail here instead, before anything is built or
+      # published, if the R2 secrets are missing.
+      - name: check R2 configuration
+        run: sh scripts/upload-r2.sh --check-config
+        env:
+          R2_ENDPOINT: ${{ secrets.R2_ENDPOINT }}
+          R2_BUCKET: ${{ secrets.R2_BUCKET }}
+          R2_ACCESS_KEY_ID: ${{ secrets.R2_ACCESS_KEY_ID }}
+          R2_SECRET_ACCESS_KEY: ${{ secrets.R2_SECRET_ACCESS_KEY }}
       - uses: actions/setup-go@v6
         with:
           go-version-file: "go.mod"
@@ -28,7 +40,7 @@ jobs:
         uses: goreleaser/goreleaser-action@v7
         with:
             distribution: goreleaser-pro
-            version: "~> v1"
+            version: "~> v2"
             args: release --nightly
         env:
           SDK_VERSION: ${{ steps.sdk_version.outputs.version }}
@@ -38,6 +50,10 @@ jobs:
           AWS_SECRET_ACCESS_KEY: ${{ secrets.AWS_SECRET_ACCESS_KEY }}
           S3_REGION: ${{ secrets.AWS_REGION }}
           S3_BUCKET: ${{ secrets.AWS_BUCKET }}
+          R2_ENDPOINT: ${{ secrets.R2_ENDPOINT }}
+          R2_BUCKET: ${{ secrets.R2_BUCKET }}
+          R2_ACCESS_KEY_ID: ${{ secrets.R2_ACCESS_KEY_ID }}
+          R2_SECRET_ACCESS_KEY: ${{ secrets.R2_SECRET_ACCESS_KEY }}
           GORELEASER_FORCE_TOKEN: 'gitea'
           GPGSIGN_PASSPHRASE: ${{ secrets.GPGSIGN_PASSPHRASE }}
           GPG_FINGERPRINT: ${{ steps.import_gpg.outputs.fingerprint }}