backend: add bookmark CRUD with public list endpoint

New bookmarks feature: domain model, repository, service and handler
supporting list/create/update/delete. Public endpoint exposes the
admin user's bookmarks for the homepage navigation grid; authenticated
endpoints scope by user. Dev Dockerfile drops air for plain `go run`
and uses goproxy.cn to avoid build failures on the deploy host.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/Dockerfile

@@ -9,14 +9,15 @@ RUN CGO_ENABLED=0 GOOS=linux go build -o server ./cmd/api/main.go
 
 FROM golang:1.25-alpine AS dev
 
-RUN go install github.com/air-verse/air@latest
+ENV GOPROXY=https://goproxy.cn,direct
+ENV GOSUMDB=off
 
 WORKDIR /app
 COPY go.mod go.sum ./
 RUN go mod download
 
 EXPOSE 8080
-CMD ["air", "-c", ".air.toml"]
+CMD ["go", "run", "./cmd/api/main.go"]
 
 FROM alpine:latest AS production