ci(release): API via GITHUB_SERVER_URL, download links via PUBLIC_HOST

Drop fragile STAGING_HOST/PROD_HOST host-detection. The runner's
GITHUB_SERVER_URL reports the internal name (gitea.xenophon), which never
matched the public-DNS vars. Now the API uses GITHUB_SERVER_URL directly
(reliable from CI) and the release-body download links use a single
per-instance PUBLIC_HOST var (gitea.scriptorium / repos.gmgauthier.com).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/release.yml

@@ -78,31 +78,24 @@ jobs:
         shell: bash
         env:
           GITEA_TOKEN: ${{ secrets.RELEASE_TOKEN }}
-          STAGING_HOST: ${{ vars.STAGING_HOST }}
-          PROD_HOST: ${{ vars.PROD_HOST }}
+          # Public DNS name of this instance, used for the user-facing download
+          # links in the release body. Set per instance, e.g.
+          #   staging: PUBLIC_HOST=gitea.scriptorium
+          #   prod:    PUBLIC_HOST=repos.gmgauthier.com
+          PUBLIC_HOST: ${{ vars.PUBLIC_HOST }}
         run: |
           set -euo pipefail
 
-          # GITHUB_SERVER_URL is injected by Gitea and points at the instance
-          # actually running this job. Strip scheme + any path to get the host.
-          SERVER_HOST=${GITHUB_SERVER_URL#http://}
-          SERVER_HOST=${SERVER_HOST#https://}
-          SERVER_HOST=${SERVER_HOST%%/*}
+          # API calls go to the instance the runner is already talking to
+          # (GITHUB_SERVER_URL — correct scheme/host/port, reachable from CI).
+          GITEA_API=${GITHUB_SERVER_URL%/}/api/v1
 
-          # Pick the configured host variable matching the running environment.
-          case "$SERVER_HOST" in
-            "$PROD_HOST")    HOST="$PROD_HOST" ;;
-            "$STAGING_HOST") HOST="$STAGING_HOST" ;;
-            *)
-              echo "ERROR: running on unrecognized host '${SERVER_HOST}'."
-              echo "Expected STAGING_HOST='${STAGING_HOST}' or PROD_HOST='${PROD_HOST}'."
-              exit 1
-              ;;
-          esac
-          echo "Detected environment host: ${HOST}"
+          # Public host for the download links baked into the release body.
+          HOST=${PUBLIC_HOST:?PUBLIC_HOST variable is not set on this Gitea instance}
+          echo "API base: ${GITEA_API}"
+          echo "Public host for release links: ${HOST}"
 
           VERSION=${GITHUB_REF#refs/tags/}
-          GITEA_API=https://${HOST}/api/v1
           REPO=${GITHUB_REPOSITORY}
 
           echo "Creating release for tag ${VERSION} on ${REPO}..."