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>

docker/Dockerfile

@@ -11,8 +11,10 @@ RUN npm run build
 FROM golang:1.22-alpine AS backend-builder
 
 WORKDIR /app/backend
-COPY backend/go.mod backend/go.sum ./
-RUN go mod download
+# Copy go.mod first for layer caching. go.sum is written by go mod download
+# when it doesn't exist yet (skeleton project), or verified when it does.
+COPY backend/go.mod ./
+RUN go mod download -x
 COPY backend/ ./
 # CGO disabled — pure Go SQLite (modernc.org/sqlite).
 RUN CGO_ENABLED=0 go build -ldflags="-s -w" -o /archivum ./cmd/server