feat: add v2 stack (backend, runner, ui-v2) with release workflow

- backend/: Go API server and runner binaries with PocketBase + MinIO storage
- ui-v2/: SvelteKit frontend rewrite
- docker-compose-new.yml: compose file for the v2 stack
- .gitea/workflows/release-v2.yaml: CI/CD for backend, runner, and ui-v2 Docker Hub images
- scripts/pb-init.sh: migrate from wget to curl, add superuser bootstrap for fresh installs
- .env.example: document DOCKER_BUILDKIT=1 for Colima users

backend/.dockerignore

@@ -0,0 +1,13 @@
+# Exclude compiled binaries
+bin/
+
+# Exclude test binaries produced by `go test -c`
+*.test
+
+# Git history is not needed inside the image
+.git/
+
+# Editor/OS noise
+.DS_Store
+*.swp
+*.swo