feat: optional centralized storage for sidecar metadata and previews (#1045)

Add an opt-in 'centralized' sidecar storage mode alongside the default
'alongside' layout. In centralized mode, .metadata.json sidecars and
preview assets live under a configurable root (sidecar_storage_path,
default <settings_dir>/sidecars), mirroring the library-relative
directory structure: <root>/<library>/<root_basename>/<rel_dir>/.

Backend:
- settings: sidecar_storage_mode / sidecar_storage_path with validation;
  changing either refreshes the preview allowlist
- config: centralized root added to preview-serving allowlist
- lifecycle: delete / move / rename / folder-rename / folder-delete and
  undoable-delete staging all operate on the mirror tree in centralized
  mode (model files themselves never move); EXDEV-tolerant cross-
  filesystem moves
- scanners: pending-hash filesystem scan walks the mirror tree in
  centralized mode; preview discovery reads from the sidecar dir;
  .civitai.info stays co-located in both modes
- migration: SidecarMigrationUseCase moves sidecars+previews between
  layouts both directions (keep-newer conflict resolution, preview_url
  rewriting, WebSocket progress), exposed as POST+GET
  /api/lm/sidecars/migrate with a mode guard (force=true for the
  settings-first flow)

Frontend:
- settings modal: sidecar storage section (mode select + path input with
  browse/validation), mode-change confirmation offering immediate
  migration (force=true), and a 'Migrate Sidecars Now' action
- i18n keys synced to all locales ([TODO: Translate] placeholders)

Docs: metadata-json-schema.md gains a storage-location section;
AGENTS.md records the sidecar_paths helper convention.

py/services/preview_asset_service.py

@@ -64,6 +64,9 @@ class PreviewAssetService:
 
         base_name = os.path.splitext(os.path.splitext(os.path.basename(metadata_path))[0])[0]
         preview_dir = os.path.dirname(metadata_path)
+        # Centralized mirrors may not exist yet (unlike the model's own
+        # directory in alongside mode).
+        os.makedirs(preview_dir, exist_ok=True)
         is_video = first_preview.get("type") == "video"
         preview_url = first_preview.get("url")
 
@@ -161,6 +164,9 @@ class PreviewAssetService:
 
         base_name = os.path.splitext(os.path.basename(model_path))[0]
         folder = get_preview_dir(model_path)
+        # Centralized mirrors may not exist yet (unlike the model's own
+        # directory in alongside mode).
+        os.makedirs(folder, exist_ok=True)
 
         extension, optimized_data = await self._convert_preview(
             preview_data, content_type, original_filename