feat: main assembly — wire all components, gin router, graceful shutdown

Dockerfile

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+FROM golang:1.22-alpine AS builder
+
+WORKDIR /app
+COPY go.mod go.sum ./
+RUN go mod download
+COPY . .
+RUN CGO_ENABLED=0 GOOS=linux go build -o /server ./cmd/server
+
+FROM alpine:3.20
+
+RUN apk add --no-cache ca-certificates tzdata
+COPY --from=builder /server /usr/local/bin/server
+COPY config/config.yaml /etc/notification/config.yaml
+
+EXPOSE 8080
+ENTRYPOINT ["server"]