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/tasks/test_runner_tasks.py

@@ -0,0 +1,62 @@
+# app/tasks/test_runner_tasks.py
+"""Background tasks for test runner."""
+
+import logging
+from datetime import UTC, datetime
+
+from app.core.database import SessionLocal
+from app.services.test_runner_service import test_runner_service
+from models.database.test_run import TestRun
+
+logger = logging.getLogger(__name__)
+
+
+async def execute_test_run(
+    run_id: int,
+    test_path: str = "tests",
+    extra_args: list[str] | None = None,
+):
+    """Background task to execute pytest tests.
+
+    Args:
+        run_id: ID of the TestRun record
+        test_path: Path to tests (relative to project root)
+        extra_args: Additional pytest arguments
+    """
+    db = SessionLocal()
+    test_run = None
+
+    try:
+        # Get the test run record
+        test_run = db.query(TestRun).filter(TestRun.id == run_id).first()
+        if not test_run:
+            logger.error(f"Test run {run_id} not found")
+            return
+
+        logger.info(f"Starting test execution: Run {run_id}, Path: {test_path}")
+
+        # Execute the tests
+        test_runner_service._execute_tests(db, test_run, test_path, extra_args)
+        db.commit()
+
+        logger.info(
+            f"Test run {run_id} completed: "
+            f"status={test_run.status}, passed={test_run.passed}, "
+            f"failed={test_run.failed}, duration={test_run.duration_seconds:.1f}s"
+        )
+
+    except Exception as e:
+        logger.error(f"Test run {run_id} failed: {e}", exc_info=True)
+        if test_run is not None:
+            try:
+                test_run.status = "error"
+                db.commit()
+            except Exception as commit_error:
+                logger.error(f"Failed to update test run status: {commit_error}")
+                db.rollback()
+    finally:
+        if hasattr(db, "close") and callable(db.close):
+            try:
+                db.close()
+            except Exception as close_error:
+                logger.error(f"Error closing database session: {close_error}")