feat: 共享配方审核完整流程

- 新增 /api/recipes/{id}/reject 端点：拒绝配方并通知提交者（含原因）
- 采纳配方时通知提交者"配方已采纳"
- 管理员拒绝配方时输入原因
- 贡献统计改为统计被采纳的配方数（含 audit_log 记录）
- 完整流程测试：共享→通知→拒绝(带原因)→通知→重新共享→采纳→通知

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/views/RecipeManager.vue

@@ -534,20 +534,28 @@ async function removeRecipe(recipe) {
 
 async function approveRecipe(recipe) {
   try {
-    await api('/api/recipes/' + recipe._id + '/adopt', { method: 'POST' })
-    ui.showToast('已采纳')
-    await recipeStore.loadRecipes()
+    const res = await api('/api/recipes/' + recipe._id + '/adopt', { method: 'POST' })
+    if (res.ok) {
+      ui.showToast('已采纳并通知提交者')
+      await recipeStore.loadRecipes()
+    }
   } catch {
     ui.showToast('操作失败')
   }
 }
 
 async function rejectRecipe(recipe) {
-  const ok = await showConfirm(`确定删除「${recipe.name}」？`)
-  if (!ok) return
+  const reason = await showPrompt(`拒绝「${recipe.name}」的原因（选填）：`)
+  if (reason === null) return
   try {
-    await recipeStore.deleteRecipe(recipe._id)
-    ui.showToast('已删除')
+    const res = await api(`/api/recipes/${recipe._id}/reject`, {
+      method: 'POST',
+      body: JSON.stringify({ reason: reason || '' }),
+    })
+    if (res.ok) {
+      await recipeStore.loadRecipes()
+      ui.showToast('已拒绝并通知提交者')
+    }
   } catch {
     ui.showToast('操作失败')
   }