fix(phase3): subscription leak, image mime type validation

- Ledger store: add stopSubscription() to properly clean up realtime
  subscriptions, matching asset store pattern
- LedgerList: call stopSubscription on unmount
- Assets migration: restrict image upload to image/* mime types
  (C3 updateRule is a known tradeoff — PocketBase lacks field-level
  permissions, frontend enforces edit restrictions instead)

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

frontend/src/stores/ledger.ts

@@ -16,6 +16,7 @@ export const useLedgerStore = defineStore('ledger', () => {
   const loading = ref(false)
   const summary = ref({ totalIncome: 0, totalExpense: 0, balance: 0 })
   const currentMonth = ref(new Date().toISOString().slice(0, 7))
+  let unsubFn: (() => Promise<void> | void) | null = null
 
   // 加载账目列表和汇总
   async function loadLedgers(groupId: string, month?: string) {
@@ -78,12 +79,19 @@ export const useLedgerStore = defineStore('ledger', () => {
 
   // 订阅实时更新
   async function startSubscription(groupId: string) {
-    await subscribeLedgers(groupId, () => {
-      // 收到变更后重新加载当前月份的数据
+    stopSubscription()
+    unsubFn = await subscribeLedgers(groupId, () => {
       loadLedgers(groupId)
     })
   }
 
+  function stopSubscription() {
+    if (unsubFn) {
+      unsubFn()
+      unsubFn = null
+    }
+  }
+
   return {
     ledgers,
     loading,
@@ -94,6 +102,7 @@ export const useLedgerStore = defineStore('ledger', () => {
     addLedger,
     editLedger,
     removeLedger,
-    startSubscription
+    startSubscription,
+    stopSubscription
   }
 })