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>

composer.json

@@ -1,7 +1,7 @@
 {
     "name": "dashboard/ticketing",
     "description": "Help desk ticketing snap-in for the Dashboard platform",
-    "version": "1.0.1",
+    "version": "1.0.2",
     "type": "library",
     "require": {
         "php": "^8.2",