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/autov3_backfill_service.py

@@ -27,6 +27,8 @@ import os
 import threading
 from typing import TYPE_CHECKING, Optional
 
+from ..utils.sidecar_paths import get_metadata_path
+
 if TYPE_CHECKING:  # pragma: no cover - type-check only; runtime imports are local
     from .model_scanner import ModelScanner
 
@@ -41,7 +43,7 @@ def _resolve_autov3(file_path: str) -> str:
     safetensors header hash. Returns ``''`` when neither is available.
     """
     try:
-        metadata_path = f"{os.path.splitext(file_path)[0]}.metadata.json"
+        metadata_path = get_metadata_path(file_path)
         if os.path.exists(metadata_path):
             with open(metadata_path, "r", encoding="utf-8") as handle:
                 payload = json.load(handle)