fix(scheduler): strip null legacy alias fields from config.json on save

SchedulerConfig.__init__ maps legacy auto_download/folder_scan keys to the
primary auto_download_after_rescan/folder_scan_enabled fields. However,
model_dump() was including auto_download=null and folder_scan=null in
serialised output. When this was written to config.json and reloaded,
those keys were present (albeit null), so the alias mapping was skipped
and the primary fields retained default False values instead of the
configured True values.

Fix:
- Override SchedulerConfig.model_dump() to drop None-valued alias fields
  before returning the serialised dict.
- ConfigService.save_config() re-serialises the scheduler field through
  its overridden model_dump() so the fix applies when writing to disk.

Tests added:
- test_roundtrip_excludes_none_alias_fields: verifies model_dump omits
  null auto_download/folder_scan keys.
- test_save_and_load_scheduler_flags_roundtrip: end-to-end roundtrip
  through ConfigService confirms raw JSON and loaded values match.

Pre-existing failure in test_core_error_handler.py is unrelated.

tests/unit/test_config_service.py

@@ -95,6 +95,37 @@ class TestConfigServiceLoadSave:
         assert loaded_config.logging.level == sample_config.logging.level
         assert loaded_config.other == sample_config.other
 
+    def test_save_and_load_scheduler_flags_roundtrip(self, config_service):
+        """Scheduler auto_download_after_rescan and folder_scan_enabled must
+        survive a full save/load roundtrip through ConfigService.
+
+        Regression test for a bug where null legacy alias fields
+        (auto_download=None, folder_scan=None) were written to config.json
+        on save. On reload the alias mapping was skipped (because the keys
+        were present), causing the primary boolean fields to reset to False.
+        """
+        original = AppConfig(
+            scheduler=SchedulerConfig(
+                enabled=True,
+                auto_download_after_rescan=True,
+                folder_scan_enabled=True,
+            )
+        )
+        config_service.save_config(original, create_backup=False)
+
+        # Verify raw JSON does not contain legacy alias keys
+        with open(config_service.config_path, "r", encoding="utf-8") as f:
+            raw = json.load(f)
+        assert "auto_download" not in raw["scheduler"]
+        assert "folder_scan" not in raw["scheduler"]
+        assert raw["scheduler"]["auto_download_after_rescan"] is True
+        assert raw["scheduler"]["folder_scan_enabled"] is True
+
+        # Verify loaded config preserves values
+        loaded = config_service.load_config()
+        assert loaded.scheduler.auto_download_after_rescan is True
+        assert loaded.scheduler.folder_scan_enabled is True
+
     def test_save_includes_version(self, config_service, sample_config):
         """Test that saved config includes version field."""
         config_service.save_config(sample_config, create_backup=False)