Add a named volume for backend build artifacts to prevent root-owned files
created inside the container from blocking host Gradle builds. This follows
the same pattern as the existing backend-gradle-project volume.
Configure OpenCode with LSP, formatter, auto-compaction, and file watcher
settings for improved development experience.
Changes:
- docker-compose.yml: add backend-build:/app/backend/build volume
- opencode.json: enable lsp, formatter, auto-compaction, prune, and
file watcher with ignore patterns for node_modules, .git, dist, build
