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/api/services/disk.service.ts

@@ -34,6 +34,12 @@ export interface CreateDiskPayload {
   set_as_default?: boolean
 }
 
+export interface DiskPurposes {
+  media_disk_id: number | null
+  pdf_disk_id: number | null
+  backup_disk_id: number | null
+}
+
 export const diskService = {
   async list(params?: ListParams): Promise<PaginatedResponse<Disk>> {
     const { data } = await client.get(API.DISKS, { params })
@@ -67,4 +73,14 @@ export const diskService = {
     const { data } = await client.get(API.DISK_DRIVERS)
     return data
   },
+
+  async getDiskPurposes(): Promise<DiskPurposes> {
+    const { data } = await client.get(API.DISK_PURPOSES)
+    return data
+  },
+
+  async updateDiskPurposes(payload: Partial<DiskPurposes>): Promise<{ success: boolean }> {
+    const { data } = await client.put(API.DISK_PURPOSES, payload)
+    return data
+  },
 }