ci: log into registry as token owner (admin), not org name

Gitea's docker login is lenient and accepts the org name as a username
at handshake time, but the per-scope token exchange that buildkit
performs for blob operations needs to resolve a real user identity.
Using admin (the user that owns REGISTRY_TOKEN) prevents 401s on
blob HEAD requests during push.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.farhoodlabs/.github/workflows/build-prod.yml

@@ -24,7 +24,7 @@ jobs:
         uses: docker/login-action@v3
         with:
           registry: git.farh.net
-          username: ${{ gitea.repository_owner }}
+          username: admin
           password: ${{ secrets.REGISTRY_TOKEN }}
 
       - name: Docker meta

.github/workflows/build-prod.yml

@@ -24,7 +24,7 @@ jobs:
         uses: docker/login-action@v3
         with:
           registry: git.farh.net
-          username: ${{ gitea.repository_owner }}
+          username: admin
           password: ${{ secrets.REGISTRY_TOKEN }}
 
       - name: Docker meta