Fix container registry login: GITEA_TOKEN never grants package access

The auto-injected secrets.GITEA_TOKEN doesn't work for pushing to
Gitea's container registry regardless of the workflow's own
permissions: block -- a known Gitea limitation, not a config mistake
(go-gitea/gitea#23642). Switch to a manually-configured
CONTAINER_REGISTRY_PASSWORD secret (a personal access token with
write:package scope) instead.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitea/workflows/build.yml

@@ -10,18 +10,18 @@ name: Build
 # This project's CI/CD scope is intentionally just test/build/push --
 # deployment is being split into a separate project.
 #
-# Requires secrets.GITEA_TOKEN (auto-injected by Gitea Actions; needs the
-# "packages" permission scope to push images).
+# Requires secrets.CONTAINER_REGISTRY_PASSWORD -- a personal access token
+# (write:package scope) from the pushing account, stored manually as a repo
+# secret. The auto-injected secrets.GITEA_TOKEN does NOT work for this: it
+# never grants package-registry access regardless of the workflow's own
+# `permissions:` block -- a known Gitea limitation, not a config mistake
+# (https://github.com/go-gitea/gitea/issues/23642).
 
 on:
   push:
     branches: [main]
   workflow_dispatch:
 
-permissions:
-  contents: read
-  packages: write
-
 jobs:
   build-and-push:
     runs-on: ubuntu-latest
@@ -42,8 +42,8 @@ jobs:
 
       - name: Log in to the container registry
         env:
-          GITEA_TOKEN: ${{ secrets.GITEA_TOKEN }}
-        run: echo "$GITEA_TOKEN" | docker login code.aneur.in -u "${{ gitea.actor }}" --password-stdin
+          CONTAINER_REGISTRY_PASSWORD: ${{ secrets.CONTAINER_REGISTRY_PASSWORD }}
+        run: echo "$CONTAINER_REGISTRY_PASSWORD" | docker login code.aneur.in -u "${{ gitea.actor }}" --password-stdin
 
       - name: Promote the current "latest" to "previous"
         run: |