fix: 扫描缓存与真实大小计算，解决页面加载慢与数据虚增

- scanner: 大小改为统计实际 .tgz 文件（兼容 {version}.tgz 与 {pkg}-{version}.tgz 命名），
  不再用 unpackedSize 兜底虚增数倍
- server: 扫描结果内存缓存（首次懒扫描，后续请求秒回），新增 POST /api/scan 手动重扫，
  /api/packages 返回 scannedAt 时间戳，clean/删版本后自动失效缓存
- 前端: 概览页新增「重新扫描」按钮 + 「基于 xxx 扫描」时间展示；Top30 在真实环境
  （无访问统计）降级为「缓存占用 Top 30」按大小排序
- compose: 数据卷改 named volume，修复非 root 容器 SQLITE_CANTOPEN

frontend/src/store/index.js

@@ -12,6 +12,7 @@ const state = reactive({
   threshold: { versionThreshold: 15, sizeThreshold: 500, exceptions: [] },
   logs: [],
   loading: false,
+  scannedAt: null,
   storageDir: localStorage.getItem('vcm-storage-dir') || '',
 })
 
@@ -23,8 +24,13 @@ const stats = computed(() => {
   const cleanable = valid.filter(p => p.status === 'warn').reduce((s, p) => s + p.size, 0)
   const maxPkg = [...valid].sort((a, b) => b.size - a.size)[0] || null
   const oldestPkg = [...valid].sort((a, b) => (a.lastUpdated || '').localeCompare(b.lastUpdated || ''))[0] || null
-  const top30 = [...valid].sort((a, b) => (b.visits || 0) - (a.visits || 0)).slice(0, 30)
+  // 真实环境无访问统计（visits 全 0）时，Top30 降级为「缓存占用 Top 30」
+  const hasVisits = valid.some(p => (p.visits || 0) > 0)
+  const top30 = [...valid]
+    .sort((a, b) => (hasVisits ? (b.visits || 0) - (a.visits || 0) : b.size - a.size))
+    .slice(0, 30)
   const maxVisit = Math.max(...top30.map(p => p.visits || 0), 1)
+  const maxSize = Math.max(...top30.map(p => p.size), 1)
   return {
     pkgCount: valid.length,
     totalSize,
@@ -34,7 +40,13 @@ const stats = computed(() => {
     cleanableText: fmtSize(cleanable),
     maxPkg,
     oldestPkg,
-    top30: top30.map(p => ({ ...p, heat: Math.round(((p.visits || 0) / maxVisit) * 100) })),
+    top30: top30.map(p => ({
+      ...p,
+      heat: hasVisits
+        ? Math.round(((p.visits || 0) / maxVisit) * 100)
+        : Math.round((p.size / maxSize) * 100),
+    })),
+    top30By: hasVisits ? 'visits' : 'size',
   }
 })
 
@@ -46,7 +58,16 @@ function fmtSize(mb) {
 const fmtSizeFn = fmtSize
 
 async function refreshPackages(keyword = '') {
-  state.packages = await api.getPackages(keyword)
+  const { packages, scannedAt } = await api.getPackages(keyword)
+  state.packages = packages
+  if (scannedAt) state.scannedAt = scannedAt
+}
+
+/** 手动重新扫描缓存目录 */
+async function scanNow() {
+  const res = await api.scanNow()
+  if (res.ok) await refreshAll()
+  return res
 }
 
 async function refreshThresholds() {
@@ -113,6 +134,7 @@ export function useStore() {
     refreshPackages,
     refreshThresholds,
     refreshLogs,
+    scanNow,
     saveThresholds,
     ignorePkg,
     unignorePkg,