fix(scheduler): skip rescan during initial setup when anime directory not configured

Prevent scheduler from triggering immediate rescan when:
- No previous scan recorded AND initial setup not yet completed
- Anime directory doesn't exist during initial sync

The setup flow will trigger rescan when ready. Also adds graceful
handling when anime directory is missing during data file sync.

Fixes: 503 error on setup when scheduler triggers rescan before
anime directory is configured

tests/unit/test_initialization_service.py

@@ -160,8 +160,11 @@ class TestSyncAnimeFolders:
     @pytest.mark.asyncio
     async def test_sync_anime_folders_without_progress(self):
         """Test syncing anime folders without progress service."""
-        with patch('src.server.services.initialization_service.sync_legacy_series_to_db', 
+        with patch('src.server.services.initialization_service.settings') as mock_settings, \
+             patch('src.server.services.initialization_service.os.path.isdir', return_value=True), \
+             patch('src.server.services.initialization_service.sync_legacy_series_to_db', 
                    new_callable=AsyncMock, return_value=42) as mock_sync:
+            mock_settings.anime_directory = "/path/to/anime"
             result = await _sync_anime_folders()
         
         assert result == 42
@@ -172,8 +175,11 @@ class TestSyncAnimeFolders:
         """Test syncing anime folders with progress updates."""
         mock_progress = AsyncMock()
         
-        with patch('src.server.services.initialization_service.sync_legacy_series_to_db', 
+        with patch('src.server.services.initialization_service.settings') as mock_settings, \
+             patch('src.server.services.initialization_service.os.path.isdir', return_value=True), \
+             patch('src.server.services.initialization_service.sync_legacy_series_to_db', 
                    new_callable=AsyncMock, return_value=10) as mock_sync:
+            mock_settings.anime_directory = "/path/to/anime"
             result = await _sync_anime_folders(progress_service=mock_progress)
         
         assert result == 10