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/adminhandler/ratelimit.go

@@ -0,0 +1,41 @@
+package adminhandler
+
+import (
+	"sync"
+	"time"
+)
+
+// rateLimiter is a simple fixed-window per-key limiter used to throttle
+// anonymous public submissions (basic anti-spam; captcha can be added later).
+type rateLimiter struct {
+	mu     sync.Mutex
+	hits   map[string][]time.Time
+	limit  int
+	window time.Duration
+}
+
+func newRateLimiter(limit int, window time.Duration) *rateLimiter {
+	return &rateLimiter{hits: map[string][]time.Time{}, limit: limit, window: window}
+}
+
+// allow reports whether the key may proceed, recording the hit if so.
+func (r *rateLimiter) allow(key string) bool {
+	now := time.Now()
+	cutoff := now.Add(-r.window)
+
+	r.mu.Lock()
+	defer r.mu.Unlock()
+
+	kept := r.hits[key][:0]
+	for _, t := range r.hits[key] {
+		if t.After(cutoff) {
+			kept = append(kept, t)
+		}
+	}
+	if len(kept) >= r.limit {
+		r.hits[key] = kept
+		return false
+	}
+	r.hits[key] = append(kept, now)
+	return true
+}