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/settings/library.html

@@ -321,4 +321,60 @@
             ('civitai_sqlite_archive', 'settings.metadataArchive.providerOrderCivitaiSqliteArchive'),
         ], 'settings.metadataArchive.providerOrderHelp') }}
     </div>
+
+    <!-- Sidecar Storage -->
+    <div class="settings-subsection">
+        {{ sm.subsection_header('settings.sections.sidecarStorage') }}
+        <div class="setting-item">
+            <div class="setting-row">
+                <div class="setting-info">
+                    <label for="sidecarStorageMode">
+                        {{ t('settings.sidecarStorage.mode') }}
+                        <i class="fas fa-info-circle info-icon" data-tooltip="{{ t('settings.sidecarStorage.modeHelp') }}"></i>
+                    </label>
+                </div>
+                <div class="setting-control select-control">
+                    <select id="sidecarStorageMode" onchange="settingsManager.handleSidecarStorageModeChange()">
+                        <option value="alongside">{{ t('settings.sidecarStorage.modeOptions.alongside') }}</option>
+                        <option value="centralized">{{ t('settings.sidecarStorage.modeOptions.centralized') }}</option>
+                    </select>
+                </div>
+            </div>
+        </div>
+
+        <div class="setting-item" id="sidecarStoragePathSetting" style="display: none;">
+            <div class="setting-row">
+                <div class="setting-info">
+                    <label for="sidecarStoragePath">
+                        {{ t('settings.sidecarStorage.path') }}
+                        <i class="fas fa-info-circle info-icon" data-tooltip="{{ t('settings.sidecarStorage.pathHelp') }}"></i>
+                    </label>
+                </div>
+                <div class="setting-control">
+                    <div class="text-input-wrapper">
+                        <input type="text" id="sidecarStoragePath"
+                               placeholder="{{ t('settings.sidecarStorage.pathPlaceholder') }}"
+                               onblur="settingsManager.saveInputSetting('sidecarStoragePath', 'sidecar_storage_path')"
+                               onkeydown="if(event.key === 'Enter') { this.blur(); }" />
+                    </div>
+                </div>
+            </div>
+        </div>
+
+        <div class="setting-item">
+            <div class="setting-row">
+                <div class="setting-info">
+                    <label>
+                        {{ t('settings.sidecarStorage.management') }}
+                        <i class="fas fa-info-circle info-icon" data-tooltip="{{ t('settings.sidecarStorage.managementHelp') }}"></i>
+                    </label>
+                </div>
+                <div class="setting-control">
+                    <button type="button" id="migrateSidecarsBtn" class="primary-btn" onclick="settingsManager.confirmAndMigrateSidecars()">
+                        {{ t('settings.sidecarStorage.migrateButton') }}
+                    </button>
+                </div>
+            </div>
+        </div>
+    </div>
 </div>