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/models/config.py

@@ -44,6 +44,18 @@ class SchedulerConfig(BaseModel):
         description="Run folder maintenance (NFO repair, folder renaming, "
                     "poster checks) during the scheduled run.",
     )
+    # Legacy alias fields — read via Pydantic alias
+    auto_download: Optional[bool] = Field(default=None, alias="auto_download")
+    folder_scan: Optional[bool] = Field(default=None, alias="folder_scan")
+
+    def __init__(self, **data):
+        super().__init__(**data)
+        # Map legacy keys to primary fields only when primary key absent from data.
+        # "key in data" checks for explicit presence (even False/None), not just truthiness.
+        if self.auto_download is not None and "auto_download_after_rescan" not in data:
+            object.__setattr__(self, "auto_download_after_rescan", self.auto_download)
+        if self.folder_scan is not None and "folder_scan_enabled" not in data:
+            object.__setattr__(self, "folder_scan_enabled", self.folder_scan)
 
     @field_validator("schedule_time")
     @classmethod
@@ -69,6 +81,22 @@ class SchedulerConfig(BaseModel):
             )
         return v
 
+    def model_dump(self, **kwargs) -> Dict[str, object]:
+        """Serialize, excluding legacy alias fields when they are None.
+
+        The alias fields (auto_download, folder_scan) must not be written to
+        config.json as null entries, otherwise a roundtrip load sees the key
+        present (哪怕 value is None) and skips the alias-to-primary mapping.
+        """
+        data = super().model_dump(**kwargs)
+        # Drop None alias fields so they don't pollute config.json.
+        # They are still settable via the constructor for backward compatibility.
+        if data.get("auto_download") is None:
+            data.pop("auto_download", None)
+        if data.get("folder_scan") is None:
+            data.pop("folder_scan", None)
+        return data
+
 
 class BackupConfig(BaseModel):
     """Configuration for automatic backups of application data."""