fix: Dockerfile no longer requires go.sum to exist before first build

Removed go.sum from the COPY cache layer — go mod download -x creates it
when absent. Added empty backend/go.sum for local tooling (gopls, go build).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -22,6 +22,8 @@ config.json
 settings.json
 *.db
 
+# go.sum is intentionally committed once populated by go mod tidy
+
 # OS
 .DS_Store
 Thumbs.db