Refactor FileDisk system with per-disk unique names and disk assignments UI

Major changes to the file disk subsystem:

- Each FileDisk now gets a unique Laravel disk name (disk_{id}) instead
  of temp_{driver}, fixing the bug where multiple local disks with
  different roots overwrote each other's config.

- Move disk registration logic from FileDisk model to FileDiskService
  (registerDisk, getDiskName). Model keeps only getDecodedCredentials
  and a deprecated setConfig() wrapper.

- Add Disk Assignments admin UI (File Disk tab) with three purpose
  dropdowns: Media Storage, PDF Storage, Backup Storage. Stored as
  settings (media_disk_id, pdf_disk_id, backup_disk_id).

- Backup tab now uses the assigned backup disk instead of a per-backup
  dropdown. BackupsController refactored to use BackupService which
  centralizes disk resolution. Removed stale 4-second cache.

- Add local_public disk to config/filesystems.php so system disks
  are properly defined.

- Local disk roots stored relative to storage/app/ with hint text
  in the admin modal explaining the convention.

- Fix BaseModal watchEffect -> watch to prevent infinite request
  loops on the File Disk page.

- Fix string/number comparison for disk purpose IDs from settings.

- Add safeguards: prevent deleting disks with files, warn on
  purpose change, prevent deleting system disks.

resources/scripts-v2/components/base/BaseModal.vue

@@ -93,7 +93,7 @@
 
 <script setup lang="ts">
 import { useModalStore } from '@v2/stores/modal.store'
-import { computed, watchEffect, useSlots } from 'vue'
+import { computed, watch, useSlots } from 'vue'
 import {
   Dialog,
   DialogOverlay,
@@ -120,9 +120,9 @@ const emit = defineEmits<Emits>()
 
 const modalStore = useModalStore()
 
-watchEffect(() => {
-  if (props.show) {
-    emit('open', props.show)
+watch(() => props.show, (newVal) => {
+  if (newVal) {
+    emit('open', newVal)
   }
 })