Pin node images to the checked-out source tree (node-26#19)

Nodes checked out v1 code but pulled :latest/:stable images built from main.
Each image is now tagged src-<git tree hash of its build directory>: CI adds
that tag in every build-*.yml, and scripts/image-tags.sh computes the same
value on the node and writes it into .env as DRB_TAG_*, which
docker-compose.yml uses (falls back to :latest/:stable for local dev).
Tree hashes stay exact even though each image only rebuilds when its own
directory changes.

- install.sh pins after checkout and fails loudly if a pinned image is
  missing (compose pull only warns for services with a build: section).
- drb-update.sh pulls the target commit's pinned images BEFORE checkout,
  using the target's own compose file, so a missing image changes nothing
  (uvicorn --reload would otherwise run new code on old images). It then
  checks out, writes the pins, recreates, and removes superseded src- tags.
- New "Release images" workflow (v* tags + manual dispatch) builds any
  src- image a release ref is missing; it skips ones already published.

The live v1 tag is not moved.

Refs #19

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitea/workflows/build-edge-node.yml

@@ -38,6 +38,10 @@ jobs:
         run: |
           echo "REPO_NAME=$(echo ${GITHUB_REPOSITORY} | awk -F'/' '{print $2}')" >> $GITHUB_OUTPUT
           echo "VERSION=$(git describe --tags --always | sed 's/^v//')" >> $GITHUB_OUTPUT
+          # Source pin (node-26#19): the git tree hash of the build context.
+          # A node pulls src-<tree of its own checkout> — scripts/image-tags.sh
+          # computes the identical value — so its code and image always match.
+          echo "SRC_TAG=src-$(git rev-parse HEAD:drb-edge-node)" >> $GITHUB_OUTPUT
 
       - uses: docker/build-push-action@v6
         with:
@@ -47,6 +51,7 @@ jobs:
           push: true
           tags: |
             git.vpn.cusano.net/${{ vars.DOCKER_ORG }}/${{ steps.meta.outputs.REPO_NAME }}/${{ env.CONTAINER_NAME }}:${{ steps.meta.outputs.VERSION }}
+            git.vpn.cusano.net/${{ vars.DOCKER_ORG }}/${{ steps.meta.outputs.REPO_NAME }}/${{ env.CONTAINER_NAME }}:${{ steps.meta.outputs.SRC_TAG }}
             git.vpn.cusano.net/${{ vars.DOCKER_ORG }}/${{ steps.meta.outputs.REPO_NAME }}/${{ env.CONTAINER_NAME }}:latest
           cache-from: type=registry,ref=git.vpn.cusano.net/${{ vars.DOCKER_ORG }}/${{ steps.meta.outputs.REPO_NAME }}/${{ env.CONTAINER_NAME }}:buildcache
           cache-to: type=registry,ref=git.vpn.cusano.net/${{ vars.DOCKER_ORG }}/${{ steps.meta.outputs.REPO_NAME }}/${{ env.CONTAINER_NAME }}:buildcache,mode=max