Read the attachment disk from config, not env (secrets S3)

The three reads were `config('ticketing.storage_disk', env('TICKETING_STORAGE_DISK', 'local'))`,
but this package had no config file at all — so the config() lookup was always
null and env() was the only reader.

Once `config:cache` runs in production, a runtime env() returns NULL. Every
ticket attachment would have silently gone to the 'local' disk, ignoring
whatever the operator had configured — and previously-stored attachments would
404 because show() looked on the wrong disk.

Verified rather than assumed: with TICKETING_STORAGE_DISK=s3proof set,
config() returned 's3proof' both cached and uncached, while env() returned
's3proof' uncached and NULL cached.

Adds config/ticketing.php + mergeConfigFrom so the value is captured at
cache time, and drops the env() fallbacks.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/Http/Controllers/TicketController.php

@@ -322,7 +322,7 @@ class TicketController extends Controller
         ]);
 
         if ($request->hasFile('attachments')) {
-            $disk = config('ticketing.storage_disk', env('TICKETING_STORAGE_DISK', 'local'));
+            $disk = config('ticketing.storage_disk', 'local');
             foreach ($request->file('attachments') as $file) {
                 if (!$file->isValid()) continue;
                 $path = $file->store('ticketing/attachments/' . $ticket->id, $disk);