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.

app/Models/FileDisk.php

@@ -2,6 +2,7 @@
 
 namespace App\Models;
 
+use App\Services\FileDiskService;
 use Carbon\Carbon;
 use Illuminate\Database\Eloquent\Factories\HasFactory;
 use Illuminate\Database\Eloquent\Model;
@@ -31,6 +32,21 @@ class FileDisk extends Model
         $this->attributes['credentials'] = json_encode($value);
     }
 
+    /**
+     * Decode credentials, handling double-encoded JSON from legacy data.
+     */
+    public function getDecodedCredentials(): Collection
+    {
+        $decoded = json_decode($this->credentials, true);
+
+        // Handle double-encoded JSON (string inside string)
+        if (is_string($decoded)) {
+            $decoded = json_decode($decoded, true);
+        }
+
+        return collect($decoded ?? []);
+    }
+
     public function scopeWhereOrder($query, $orderByField, $orderBy)
     {
         $query->orderBy($orderByField, $orderBy);
@@ -83,14 +99,14 @@ class FileDisk extends Model
 
     /**
      * Apply this disk's credentials to the filesystem configuration at runtime.
+     *
+     * @deprecated Use FileDiskService::registerDisk() instead — setConfig() mutates filesystems.default.
      */
     public function setConfig(): void
     {
-        $driver = $this->driver;
-
-        $credentials = collect(json_decode($this['credentials']));
-
-        self::setFilesystem($credentials, $driver);
+        $service = app(FileDiskService::class);
+        $diskName = $service->registerDisk($this);
+        config(['filesystems.default' => $diskName]);
     }
 
     /**
@@ -103,6 +119,8 @@ class FileDisk extends Model
 
     /**
      * Register a dynamic filesystem disk in the runtime configuration using the given credentials.
+     *
+     * @deprecated Use FileDisk::find($id)->registerDisk() instead.
      */
     public static function setFilesystem(Collection $credentials, string $driver): void
     {
@@ -118,6 +136,10 @@ class FileDisk extends Model
             }
         }
 
+        if ($driver === 'local' && isset($disks['root']) && ! str_starts_with($disks['root'], '/')) {
+            $disks['root'] = storage_path('app/'.$disks['root']);
+        }
+
         config(['filesystems.disks.'.$prefix.$driver => $disks]);
     }