feat(server): main.go assemblage complet + Docker + Nginx

- Assemble tous les packages (config, db, handlers, transport, websocket, prometheus)
- Boucle de rétention et détection offline toutes les minutes
- Routage REST /api/agents/, /api/config, /metrics, /ws
- Dockerfile multi-stage CGO_ENABLED=0 (alpine:3.19)
- docker-compose.yml avec service server + dashboard Nginx
- nginx.conf avec proxy WebSocket et fallback SPA

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

server/Dockerfile

@@ -0,0 +1,14 @@
+FROM golang:1.22-alpine AS builder
+WORKDIR /app
+COPY go.mod go.sum ./
+RUN go mod download
+COPY . .
+RUN CGO_ENABLED=0 go build -ldflags="-s -w" -o nanometrics-server .
+
+FROM alpine:3.19
+RUN apk add --no-cache ca-certificates
+WORKDIR /app
+COPY --from=builder /app/nanometrics-server .
+VOLUME /data
+EXPOSE 8080 9999/udp
+CMD ["./nanometrics-server"]