ci: filter main builds to code changes and cancel superseded runs

Apply path filters to the push (merge-to-main) trigger only, so merges that
do not change the image (CI/Renovate config, docs) skip the build and release.
Pull requests keep running unfiltered so the required check is always reported.

Add a concurrency group keyed by ref with cancel-in-progress on main, so a
newer merge cancels an older in-flight build and only the most recent release
is produced, avoiding tags that would be immediately replaced.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitea/workflows/build-and-publish.yml

@@ -1,10 +1,29 @@
 name: Build and publish container
 
 on:
+  # On merge to main, only build/release when image-affecting files change;
+  # CI-config, Renovate-config and docs changes do not produce a new image.
   push:
     branches: [main]
+    paths:
+      - 'Dockerfile'
+      - 'default.conf'
+      - 'index.html'
+      - 'styles.css'
+      - 'script.js'
+      - 'messages.js'
+      - '.dockerignore'
+  # Pull requests always run (the build is a required check); no path filter.
   pull_request:
     branches: [main]
+  workflow_dispatch:
+
+# On main, a newer merge cancels an older in-flight build so only the latest
+# release is produced, avoiding tags that would be immediately replaced.
+# Pull request runs are left to finish (cancel only on main).
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: ${{ github.ref == 'refs/heads/main' }}
 
 defaults:
   run: