feat: 公开首页(搜索+商品详情) + 好心人投稿 + 后台审核收纳

- 公开前端 SPA（根路径 /）：首页大搜索框、检索结果、只读商品详情、贡献档案表单
- 公开写入端点 POST /api/public/submissions（无需登录，基础频率限流），投稿进入 submission 待审核队列，不直接写 product
- 迁移 0006：submission 投稿表 + community 来源(trust=0.50)
- 后台审核队列：列表(待审核/已通过/已驳回) → 查看投稿 → 通过(创建/补全商品 + 记 source=community + 字段级溯源 + 审计 + 重算质量分) / 驳回(记原因)
- 公开只读 api 服务内嵌公开 SPA；Dockerfile.prod 增加 node 构建阶段 + 内嵌 dist

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

api/internal/publicweb/embed.go

@@ -0,0 +1,22 @@
+// Package publicweb embeds the built public SPA (Vite dist). During Docker
+// builds the real dist/ is produced by the node stage and copied in before go
+// build; the committed placeholder keeps the package compilable for
+// `go build ./...`.
+package publicweb
+
+import (
+	"embed"
+	"io/fs"
+)
+
+//go:embed all:dist
+var distFS embed.FS
+
+// Dist returns the embedded SPA filesystem rooted at dist/.
+func Dist() fs.FS {
+	sub, err := fs.Sub(distFS, "dist")
+	if err != nil {
+		panic(err)
+	}
+	return sub
+}