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/components/ledger/LedgerList.vue

@@ -1,5 +1,5 @@
 <script setup lang="ts">
-import { ref, computed, onMounted, watch } from 'vue'
+import { ref, computed, onMounted, onUnmounted, watch } from 'vue'
 import { ElMessage, ElMessageBox } from 'element-plus'
 import { Plus } from '@element-plus/icons-vue'
 import { useLedgerStore } from '@/stores/ledger'
@@ -139,6 +139,10 @@ onMounted(() => {
   }
 })
 
+onUnmounted(() => {
+  ledgerStore.stopSubscription()
+})
+
 watch(() => groupStore.currentGroupId, (newId, oldId) => {
   if (newId && newId !== oldId) {
     loadData()