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

src/server/services/initialization_service.py

@@ -215,6 +215,20 @@ async def _sync_anime_folders(progress_service=None) -> int:
     """
     logger.info("Performing initial anime folder scan...")
     
+    # Check if anime directory exists before attempting sync
+    if not settings.anime_directory or not os.path.isdir(settings.anime_directory):
+        logger.info(
+            "Anime directory not configured or does not exist, skipping data file sync"
+        )
+        if progress_service:
+            await progress_service.update_progress(
+                progress_id="series_sync",
+                current=100,
+                message="No anime directory configured, skipping data file sync",
+                metadata={"step_id": "series_sync"}
+            )
+        return 0
+    
     if progress_service:
         await progress_service.update_progress(
             progress_id="series_sync",