Implement Phase 10: deployment (Dockerfile, systemd, install script)

- Multi-stage Dockerfile: golang:1.25-alpine builder, alpine:3.21 runtime
  CGO_ENABLED=0, stripped binary, non-root user
- systemd: service unit (hardened), backup oneshot, daily timer (02:00 UTC)
- Install script: create user, dirs, config, install units
- Updated PROGRESS.md with all completed phases

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

Dockerfile

@@ -0,0 +1,16 @@
+FROM golang:1.25-alpine AS builder
+WORKDIR /src
+COPY go.mod go.sum ./
+RUN go mod download
+COPY . .
+RUN CGO_ENABLED=0 go build -trimpath -ldflags="-s -w" -o /eng-pad-server ./cmd/eng-pad-server
+
+FROM alpine:3.21
+RUN apk add --no-cache ca-certificates && \
+    adduser -D -h /srv/eng-pad-server engpad
+USER engpad
+WORKDIR /srv/eng-pad-server
+COPY --from=builder /eng-pad-server /usr/local/bin/eng-pad-server
+EXPOSE 8443 9443 8080
+ENTRYPOINT ["eng-pad-server"]
+CMD ["server", "-c", "/srv/eng-pad-server/eng-pad-server.toml"]