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/api/v1/admin/tests.py

@@ -3,13 +3,14 @@ Test Runner API Endpoints
 RESTful API for running pytest and viewing test results
 """
 
-from fastapi import APIRouter, Depends, Query
+from fastapi import APIRouter, BackgroundTasks, Depends, Query
 from pydantic import BaseModel, Field
 from sqlalchemy.orm import Session
 
 from app.api.deps import get_current_admin_api
 from app.core.database import get_db
 from app.services.test_runner_service import test_runner_service
+from app.tasks.test_runner_tasks import execute_test_run
 from models.database.user import User
 
 router = APIRouter()
@@ -96,27 +97,37 @@ class TestDashboardStatsResponse(BaseModel):
 
 @router.post("/run", response_model=TestRunResponse)
 async def run_tests(
+    background_tasks: BackgroundTasks,
     request: RunTestsRequest | None = None,
     db: Session = Depends(get_db),
     current_user: User = Depends(get_current_admin_api),
 ):
     """
-    Run pytest and store results
+    Start a pytest run in the background
 
-    Requires admin authentication. Runs pytest on the specified path
-    and stores results in the database.
+    Requires admin authentication. Creates a test run record and starts
+    pytest execution in the background. Returns immediately with the run ID.
+    Poll GET /runs/{run_id} to check status.
     """
     test_path = request.test_path if request else "tests"
     extra_args = request.extra_args if request else None
 
-    run = test_runner_service.run_tests(
+    # Create the test run record
+    run = test_runner_service.create_test_run(
         db,
         test_path=test_path,
         triggered_by=f"manual:{current_user.username}",
-        extra_args=extra_args,
     )
     db.commit()
 
+    # Start background execution
+    background_tasks.add_task(
+        execute_test_run,
+        run.id,
+        test_path,
+        extra_args,
+    )
+
     return TestRunResponse(
         id=run.id,
         timestamp=run.timestamp.isoformat(),