test: add tests for scheduled folder scan and startup NFO repair removal

Add comprehensive test coverage for Tasks 1.1–1.5 and 2.1:

- test_scheduler_config_model.py: folder_scan_enabled defaults, explicit
  values, backward compatibility with old configs, serialization roundtrip
- test_folder_scan_service.py (new): prerequisites, NFO repair integration,
  folder rename integration, poster check/download, semaphore values,
  NFO thumb URL extraction, full end-to-end scan flow
- test_scheduler_service.py: scheduler _perform_rescan integration with
  folder_scan_enabled (called when enabled, skipped when disabled, error
  handling and broadcasting), folder_scan_enabled in get_status output
- test_nfo_repair_startup.py: verify perform_nfo_repair_scan is NOT called
  during FastAPI lifespan startup and IS called from FolderScanService

All 90 tests pass.

tests/unit/test_scheduler_config_model.py

@@ -113,8 +113,36 @@ class TestSchedulerConfigBackwardCompat:
         assert config.interval_minutes == 30
 
 
+class TestSchedulerConfigFolderScanEnabled:
+    """3.8 – folder_scan_enabled field (Task 1.1)."""
+
+    def test_default_folder_scan_enabled(self) -> None:
+        config = SchedulerConfig()
+        assert config.folder_scan_enabled is False
+
+    def test_set_folder_scan_enabled_true(self) -> None:
+        config = SchedulerConfig(folder_scan_enabled=True)
+        assert config.folder_scan_enabled is True
+
+    def test_set_folder_scan_enabled_false(self) -> None:
+        config = SchedulerConfig(folder_scan_enabled=False)
+        assert config.folder_scan_enabled is False
+
+    def test_backward_compat_missing_field(self) -> None:
+        """Old configs without folder_scan_enabled load successfully."""
+        dumped = {
+            "enabled": True,
+            "interval_minutes": 60,
+            "schedule_time": "03:00",
+            "schedule_days": ALL_DAYS,
+            "auto_download_after_rescan": False,
+        }
+        config = SchedulerConfig(**dumped)
+        assert config.folder_scan_enabled is False
+
+
 class TestSchedulerConfigSerialisation:
-    """3.8 – Serialisation roundtrip."""
+    """3.9 – Serialisation roundtrip."""
 
     def test_roundtrip(self) -> None:
         original = SchedulerConfig(
@@ -123,6 +151,7 @@ class TestSchedulerConfigSerialisation:
             schedule_time="04:30",
             schedule_days=["mon", "wed", "fri"],
             auto_download_after_rescan=True,
+            folder_scan_enabled=True,
         )
         dumped = original.model_dump()
         restored = SchedulerConfig(**dumped)