Replace asyncio.to_thread with ThreadPoolExecutor.run_in_executor

- Add ThreadPoolExecutor with 3 max workers to SeriesApp
- Replace all asyncio.to_thread calls with loop.run_in_executor
- Add shutdown() method to properly cleanup executor
- Integrate SeriesApp.shutdown() into FastAPI shutdown sequence
- Ensures proper resource cleanup on Ctrl+C (SIGINT/SIGTERM)

data/config_backups/config_backup_20260103_205109.json

@@ -0,0 +1,24 @@
+{
+  "name": "Aniworld",
+  "data_dir": "data",
+  "scheduler": {
+    "enabled": true,
+    "interval_minutes": 60
+  },
+  "logging": {
+    "level": "INFO",
+    "file": null,
+    "max_bytes": null,
+    "backup_count": 3
+  },
+  "backup": {
+    "enabled": false,
+    "path": "data/backups",
+    "keep_days": 30
+  },
+  "other": {
+    "master_password_hash": "$pbkdf2-sha256$29000$X4uRMibE.N.bM.acs1ZKSQ$88em69lhlaLiS6vcF9oqf4pCC8KBbIj/O3h4cQFwM.I",
+    "anime_directory": "/mnt/server/serien/Serien/"
+  },
+  "version": "1.0.0"
+}

data/config_backups/config_backup_20260103_205117.json

@@ -0,0 +1,24 @@
+{
+  "name": "Aniworld",
+  "data_dir": "data",
+  "scheduler": {
+    "enabled": true,
+    "interval_minutes": 60
+  },
+  "logging": {
+    "level": "INFO",
+    "file": null,
+    "max_bytes": null,
+    "backup_count": 3
+  },
+  "backup": {
+    "enabled": false,
+    "path": "data/backups",
+    "keep_days": 30
+  },
+  "other": {
+    "master_password_hash": "$pbkdf2-sha256$29000$TQlBSOndm1MKAWAMoTRGKA$a/q5miowGpjWSc71WDvqBpL9JJmuAO1FrZlCi3qwp2E",
+    "anime_directory": "/mnt/server/serien/Serien/"
+  },
+  "version": "1.0.0"
+}