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.

src/server/services/config_service.py

@@ -144,7 +144,13 @@ class ConfigService:
         # Save configuration with version
         data = config.model_dump()
         data["version"] = self.CONFIG_VERSION
-        
+
+        # Re-serialize SchedulerConfig through its overridden model_dump so
+        # that None legacy alias fields are stripped before writing to disk.
+        # Pydantic converts nested models to plain dicts in model_dump() output,
+        # so we call the override explicitly on the scheduler field.
+        data["scheduler"] = config.scheduler.model_dump()
+
         # Write to temporary file first for atomic operation
         temp_path = self.config_path.with_suffix(".tmp")
         try: