refactor: route sidecar/preview path derivation through sidecar_paths helpers

Phase 1 of #1045 (optional centralized sidecar storage): introduce
py/utils/sidecar_paths.py as the single place that resolves .metadata.json
and preview locations, and replace all inline splitext-based derivations
across scanners, services, download manager, and route handlers.

No behavior change: the default 'alongside' storage mode resolves every
path exactly as before. .civitai.info (third-party sidecar) derivation is
intentionally left co-located.

py/services/use_cases/filename_template_use_case.py

@@ -12,6 +12,7 @@ import os
 from typing import Any, Awaitable, Callable, Dict, List, Optional, Sequence
 
 from ...utils.constants import AUTO_ORGANIZE_BATCH_SIZE
+from ...utils.sidecar_paths import get_metadata_path
 from ...utils.utils import calculate_filename_for_model
 from ..model_file_service import AutoOrganizeResult, ProgressCallback
 from ..model_lifecycle_service import ModelLifecycleService, load_local_metadata
@@ -200,7 +201,7 @@ class FilenameTemplateUseCase:
         sidecar or no ``original_file_name`` entry exists (models never
         renamed, or renamed before the recording shipped).
         """
-        metadata_path = f"{os.path.splitext(file_path)[0]}.metadata.json"
+        metadata_path = get_metadata_path(file_path)
         metadata = await self._metadata_loader(metadata_path)
         original = metadata.get("original_file_name")
         if not isinstance(original, str):