feat: show Jobs tab for all vendors when no filter selected

- Add /admin/letzshop/jobs API endpoint for all jobs across vendors
- Update list_letzshop_jobs service to support optional vendor_id
- Remove x-if condition from Jobs tab button and panel
- Update JS to use global or vendor-specific endpoint based on selection
- Update jobs table subtitle to show context

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/templates/admin/partials/letzshop-jobs-table.html

@@ -6,7 +6,10 @@
         <div class="flex items-center justify-between mb-4">
             <div>
                 <h3 class="text-lg font-semibold text-gray-700 dark:text-gray-200">Recent Jobs</h3>
-                <p class="text-sm text-gray-500 dark:text-gray-400">Product imports, exports, and order sync history</p>
+                <p class="text-sm text-gray-500 dark:text-gray-400">
+                    <span x-show="selectedVendor">Product imports, exports, and order sync history</span>
+                    <span x-show="!selectedVendor">All Letzshop jobs across all vendors</span>
+                </p>
             </div>
             <button
                 @click="loadJobs()"