chore: add Dockerfile for Go backend

Multi-stage build that compiles server, CLI, and migrate binaries,
then produces a minimal Alpine runtime image.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -0,0 +1,36 @@
+# --- Build stage ---
+FROM golang:1.26-alpine AS builder
+
+RUN apk add --no-cache git
+
+WORKDIR /src
+
+# Cache dependencies
+COPY server/go.mod server/go.sum ./server/
+RUN cd server && go mod download
+
+# Copy server source
+COPY server/ ./server/
+
+# Build binaries
+ARG VERSION=dev
+ARG COMMIT=unknown
+RUN cd server && CGO_ENABLED=0 go build -ldflags "-s -w" -o bin/server ./cmd/server
+RUN cd server && CGO_ENABLED=0 go build -ldflags "-s -w -X main.version=${VERSION} -X main.commit=${COMMIT}" -o bin/multica ./cmd/multica
+RUN cd server && CGO_ENABLED=0 go build -ldflags "-s -w" -o bin/migrate ./cmd/migrate
+
+# --- Runtime stage ---
+FROM alpine:3.21
+
+RUN apk add --no-cache ca-certificates tzdata
+
+WORKDIR /app
+
+COPY --from=builder /src/server/bin/server .
+COPY --from=builder /src/server/bin/multica .
+COPY --from=builder /src/server/bin/migrate .
+COPY server/migrations/ ./migrations/
+
+EXPOSE 8080
+
+ENTRYPOINT ["./server"]