flutter 상위 구조로 포함. infra 분리 리팩토링

backend/Dockerfile

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+FROM golang:1.21-alpine
+
+WORKDIR /app
+
+# Install git for go mod download if needed
+RUN apk add --no-cache git
+
+# Pre-copy go.mod/sum to cache dependencies
+COPY go.mod go.sum ./
+RUN go mod download
+
+# Copy source
+COPY . .
+
+# Build for production (optional, can just run go run for dev)
+RUN go build -o main ./cmd/server
+
+EXPOSE 3000
+
+# Default command (can be overridden by compose)
+CMD ["./main"]