feat(deploy): multi-stage Dockerfile (web build -> go embed -> alpine runtime)

Dockerfile

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+# syntax=docker/dockerfile:1.7
+
+# ─── Stage 1: web build ───
+FROM node:22-alpine AS web
+WORKDIR /web
+COPY web/package.json web/pnpm-lock.yaml* ./
+RUN corepack enable && (pnpm install --frozen-lockfile || pnpm install)
+COPY web .
+RUN pnpm build
+
+# ─── Stage 2: go build ───
+FROM golang:1.25-alpine AS go
+WORKDIR /src
+RUN apk add --no-cache git
+COPY go.mod go.sum ./
+RUN go mod download
+COPY . .
+COPY --from=web /web/dist/. ./cmd/server/web/dist/
+RUN CGO_ENABLED=0 GOOS=linux go build -trimpath -ldflags='-s -w' -o /out/server ./cmd/server
+
+# ─── Stage 3: runtime ───
+FROM alpine:3.20
+RUN apk add --no-cache ca-certificates tzdata && \
+    addgroup -S app && adduser -S app -G app
+WORKDIR /app
+COPY --from=go /out/server /app/server
+COPY migrations /app/migrations
+USER app
+EXPOSE 8080
+ENTRYPOINT ["/app/server"]