Run go test inside Docker build instead of separate docker run step

Bind-mounted source causes Go module resolution issues in the runner
container. Tests now run as a Dockerfile layer (after go mod download,
before SPA inject + go build) — no bind mount, no extra step needed.
A failed test aborts the build and the deploy never happens.

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

Dockerfile

@@ -12,6 +12,7 @@ WORKDIR /app
 COPY backend/go.mod backend/go.sum ./
 RUN go mod download
 COPY backend/ ./
+RUN go test ./...
 # Inject built SPA into embed path
 COPY --from=webapp-builder /webapp/dist ./internal/api/webapp/
 RUN CGO_ENABLED=0 GOOS=linux go build -o /server ./cmd/server