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.