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/download_manager.py

@@ -2465,6 +2465,10 @@ class DownloadManager:
             # Download preview image if available
             images = version_info.get("images", [])
             if images:
+                # Centralized preview mirrors may not exist yet (unlike the
+                # model's own directory in alongside mode).
+                os.makedirs(get_preview_dir(save_path), exist_ok=True)
+
                 if progress_callback:
                     await progress_callback(
                         1
@@ -3067,10 +3071,12 @@ class DownloadManager:
 
         first_target = targets[0]
         if preview_path != first_target:
+            os.makedirs(os.path.dirname(first_target), exist_ok=True)
             os.replace(preview_path, first_target)
         source_path = first_target
 
         for target in targets[1:]:
+            os.makedirs(os.path.dirname(target), exist_ok=True)
             shutil.copyfile(source_path, target)
 
         return targets