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.

templates/components/modals/confirm_modals.html

@@ -96,6 +96,20 @@
     </div>
 </div>
 
+<!-- Sidecar Migration Confirmation Modal
+     Self-managed by SettingsManager (NOT registered with ModalManager): it
+     stacks above the settings modal, like the directory picker. -->
+<div id="sidecarMigrationConfirmModal" class="modal delete-modal">
+    <div class="modal-content delete-modal-content">
+        <h2 data-role="title"></h2>
+        <p class="delete-message" data-role="message"></p>
+        <div class="modal-actions">
+            <button class="cancel-btn" data-action="cancel-sidecar-migration">{{ t('common.actions.cancel') }}</button>
+            <button class="primary-btn" data-action="confirm-sidecar-migration"></button>
+        </div>
+    </div>
+</div>
+
 <!-- Sidebar Folder Delete Confirmation Modal -->
 <!-- Shared by two states: 'confirm' (model-free folder) and 'blocked' (the
      subtree still holds models, so a cascade delete is refused). -->