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>

.goreleaser.yaml

@@ -1,3 +1,5 @@
+version: 2
+
 before:
   hooks:
     - go mod tidy
@@ -79,11 +81,50 @@ blobs:
     provider: s3
     bucket: "{{ .Env.S3_BUCKET }}"
     region: "{{ .Env.S3_REGION }}"
-    folder: "tea/{{.Version}}"
+    directory: "tea/{{.Version}}"
     extra_files:
       - glob: ./**.xz
       - glob: ./**.sha256
 
+# Mirrors the S3 `blobs:` upload above into Cloudflare R2 during the
+# parallel S3+R2 period (S3 will be removed once migration completes).
+# A second `blobs:` entry is impossible here since the blob pipe
+# authenticates from the global AWS_* env with no per-entry
+# credentials; `publishers:` supports per-entry `env:` instead, so
+# it's used to invoke scripts/upload-r2.sh once per artifact. Custom
+# publishers inherit almost nothing from the environment, hence the
+# explicit R2_* forwarding below.
+#
+# This publisher fires more than once per distinct key because
+# goreleaser's release pipe already registers `release.extra_files`
+# (./**.xz and ./**.xz.sha256, see the `release:` block below) as
+# UploadableFile artifacts, and `internal/exec`'s filterArtifacts
+# appends this block's own extra_files with no de-duplication. It
+# can't be globbed away, since gobwas/glob (via goreleaser/fileglob)
+# has no substring-exclusion matcher. It's harmless: PUT is
+# idempotent, and the `./**.xz` glob below is kept deliberately so
+# this publisher declares its own complete file set rather than
+# implicitly depending on the `release:` block's globs.
+#
+# checksum: true mirrors goreleaser's generated checksums.txt;
+# signature: true additionally mirrors checksums.txt.sig, which the
+# `signs:` block below produces by GPG-signing that checksum file.
+# Without signature: true, artifacts downloaded from the R2 mirror
+# would have no signature file to verify against.
+publishers:
+  - name: cloudflare-r2
+    checksum: true
+    signature: true
+    extra_files:
+      - glob: ./**.xz
+      - glob: ./**.sha256
+    cmd: sh scripts/upload-r2.sh {{ abs .ArtifactPath }} tea/{{ .Version }}/{{ .ArtifactName }}
+    env:
+      - R2_ENDPOINT={{ index .Env "R2_ENDPOINT" }}
+      - R2_BUCKET={{ index .Env "R2_BUCKET" }}
+      - R2_ACCESS_KEY_ID={{ index .Env "R2_ACCESS_KEY_ID" }}
+      - R2_SECRET_ACCESS_KEY={{ index .Env "R2_SECRET_ACCESS_KEY" }}
+
 archives:
   - format: binary
     name_template: "{{ .Binary }}"
@@ -104,10 +145,10 @@ signs:
     args: ["--batch", "-u", "{{ .Env.GPG_FINGERPRINT }}", "--output", "${signature}", "--detach-sign", "${artifact}"]
 
 snapshot:
-  name_template: "{{ .Branch }}-devel"
+  version_template: "{{ .Branch }}-devel"
 
 nightly:
-  name_template: "{{ .Branch }}"
+  version_template: "{{ .Branch }}"
 
 gitea_urls:
   api: https://gitea.com/api/v1