feat: add cancel, pause, and resume flow control for scans

Users running large scans can now pause (keep partial results, resume
later), cancel (stop permanently, partial results preserved), or resume
a paused scan which races through cache hits before continuing.

Backend:
- Extend scans.status CHECK to include 'paused' and 'cancelled'
- Add _migrate_add_pause_cancel_status() table-recreation migration
- scan_processor: _running_tasks/_cancel_reasons registries,
  cancel_scan_task/pause_scan_task/stop_scan_task helpers,
  CancelledError handler in process_scan(), start_resume_processor()
- api_server: POST /scans/{id}/pause|cancel|resume endpoints with
  rate limits (30/min pause+cancel, 10/min resume); list_scans now
  accepts paused/cancelled as status filter values

Frontend:
- Scan.status type extended with 'paused' | 'cancelled'
- scanApi.pause/cancel/resume added
- StatusChip: amber PauseCircle chip for paused, grey Ban for cancelled
- ScanDetails: context-aware action row with inline-confirm for
  Pause and Cancel; Resume button for paused scans

Tests: 129 total (58 new) across test_scan_control.py,
test_scan_processor_control.py, and additions to existing suites

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

flight-comparator/tests/test_api_endpoints.py

@@ -245,6 +245,45 @@ class TestScanEndpoints:
         assert data["data"][0]["destination"] == "FRA"
         assert data["data"][0]["min_price"] == 50
 
+    def test_get_scan_paused_status(self, client: TestClient, create_test_scan):
+        """Test that GET /scans/{id} returns paused status correctly."""
+        scan_id = create_test_scan(status='paused')
+        response = client.get(f"/api/v1/scans/{scan_id}")
+        assert response.status_code == 200
+        assert response.json()["status"] == "paused"
+
+    def test_get_scan_cancelled_status(self, client: TestClient, create_test_scan):
+        """Test that GET /scans/{id} returns cancelled status correctly."""
+        scan_id = create_test_scan(status='cancelled')
+        response = client.get(f"/api/v1/scans/{scan_id}")
+        assert response.status_code == 200
+        assert response.json()["status"] == "cancelled"
+
+    def test_list_scans_filter_paused(self, client: TestClient, create_test_scan):
+        """Test filtering scans by paused status."""
+        create_test_scan(status='paused')
+        create_test_scan(status='completed')
+        create_test_scan(status='running')
+
+        response = client.get("/api/v1/scans?status=paused")
+
+        assert response.status_code == 200
+        data = response.json()
+        assert len(data["data"]) == 1
+        assert data["data"][0]["status"] == "paused"
+
+    def test_list_scans_filter_cancelled(self, client: TestClient, create_test_scan):
+        """Test filtering scans by cancelled status."""
+        create_test_scan(status='cancelled')
+        create_test_scan(status='pending')
+
+        response = client.get("/api/v1/scans?status=cancelled")
+
+        assert response.status_code == 200
+        data = response.json()
+        assert len(data["data"]) == 1
+        assert data["data"][0]["status"] == "cancelled"
+
 
 @pytest.mark.unit
 @pytest.mark.api