add database backup and restore admin page

- SQLCipher-encrypted backup creation via VACUUM INTO
- Backup history with auto-pruning (keeps last 5)
- Pre-restore automatic backup for safety
- Restore from history or uploaded file
- Stats display with table breakdown
- Download hook for client-side file download
- SECRET_KEY_DB config for encryption at rest

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

assets/js/app.js

@@ -932,10 +932,25 @@ const EditorKeyboard = {
   }
 }
 
+// Hook to trigger file downloads from LiveView
+const Download = {
+  mounted() {
+    this.handleEvent("download", ({filename, content, content_type}) => {
+      const blob = new Blob([Uint8Array.from(atob(content), c => c.charCodeAt(0))], {type: content_type})
+      const url = URL.createObjectURL(blob)
+      const a = document.createElement("a")
+      a.href = url
+      a.download = filename
+      a.click()
+      URL.revokeObjectURL(url)
+    })
+  }
+}
+
 const csrfToken = document.querySelector("meta[name='csrf-token']").getAttribute("content")
 const liveSocket = new LiveSocket("/live", Socket, {
   params: {_csrf_token: csrfToken, screen_width: window.innerWidth},
-  hooks: {...colocatedHooks, ColorSync, Lightbox, CartPersist, CartDrawer, ProductImageScroll, SearchModal, MobileNavDrawer, CollectionFilters, AnalyticsInit, AnalyticsExport, ChartTooltip, Clipboard, DirtyGuard, EditorKeyboard, EditorSheet},
+  hooks: {...colocatedHooks, ColorSync, Lightbox, CartPersist, CartDrawer, ProductImageScroll, SearchModal, MobileNavDrawer, CollectionFilters, AnalyticsInit, AnalyticsExport, ChartTooltip, Clipboard, DirtyGuard, EditorKeyboard, EditorSheet, Download},
 })
 
 // Show progress bar on live navigation and form submits
@@ -956,6 +971,11 @@ window.addEventListener("phx:scroll-top", () => {
   window.scrollTo({top: 0, behavior: 'instant'})
 })
 
+// Scroll element into view (used by flash messages)
+window.addEventListener("scroll-into-view", (e) => {
+  e.target.scrollIntoView({behavior: 'smooth', block: 'nearest'})
+})
+
 // connect if there are any LiveViews on the page
 liveSocket.connect()