feat: run tests in background with progress polling

Improve the testing dashboard to run pytest in the background:

- Add background task execution using FastAPI's BackgroundTasks
- Create test_runner_tasks.py following existing background task pattern
- API now returns immediately after starting the test run
- Frontend polls for status every 2 seconds until completion
- Show running indicator with elapsed time counter
- Resume polling if user navigates away and returns while tests running
- Tests continue running even if user closes the page

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

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

app/templates/admin/testing-dashboard.html

@@ -23,6 +23,33 @@
 
 {{ alert_dynamic(type='success', message_var='successMessage', show_condition='successMessage') }}
 
+<!-- Running Indicator -->
+<div x-show="running" x-cloak class="mb-6">
+    <div class="p-4 bg-purple-100 dark:bg-purple-900 rounded-lg shadow-xs">
+        <div class="flex items-center justify-between">
+            <div class="flex items-center">
+                <svg class="animate-spin h-5 w-5 text-purple-600 dark:text-purple-400 mr-3" xmlns="http://www.w3.org/2000/svg" fill="none" viewBox="0 0 24 24">
+                    <circle class="opacity-25" cx="12" cy="12" r="10" stroke="currentColor" stroke-width="4"></circle>
+                    <path class="opacity-75" fill="currentColor" d="M4 12a8 8 0 018-8V0C5.373 0 0 5.373 0 12h4zm2 5.291A7.962 7.962 0 014 12H0c0 3.042 1.135 5.824 3 7.938l3-2.647z"></path>
+                </svg>
+                <div>
+                    <p class="font-semibold text-purple-800 dark:text-purple-200">Running tests...</p>
+                    <p class="text-sm text-purple-600 dark:text-purple-300">Tests are executing in the background. You can leave this page and come back.</p>
+                </div>
+            </div>
+            <div class="text-right">
+                <p class="text-2xl font-bold text-purple-700 dark:text-purple-300" x-text="formatDuration(elapsedTime)">0s</p>
+                <p class="text-xs text-purple-600 dark:text-purple-400">elapsed</p>
+            </div>
+        </div>
+        <div class="mt-3">
+            <div class="w-full bg-purple-200 dark:bg-purple-800 rounded-full h-1.5 overflow-hidden">
+                <div class="bg-purple-600 dark:bg-purple-400 h-1.5 rounded-full animate-pulse" style="width: 100%"></div>
+            </div>
+        </div>
+    </div>
+</div>
+
 <!-- Dashboard Content -->
 <div x-show="!loading && !error">
     <!-- Stats Cards Row 1 - Main Metrics -->