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.

locales/zh-TW.json

@@ -383,6 +383,7 @@
             "exampleImages": "範例圖片",
             "autoOrganize": "自動整理",
             "metadata": "中繼資料",
+            "sidecarStorage": "[TODO: Translate] Sidecar Storage",
             "proxySettings": "代理設定"
         },
         "nav": {
@@ -789,6 +790,27 @@
             "providerOrderCivitaiArchiveSqlite": "CivitAI → CivArchive → Archive DB",
             "providerOrderCivitaiSqliteArchive": "CivitAI → Archive DB → CivArchive"
         },
+        "sidecarStorage": {
+            "mode": "[TODO: Translate] Sidecar Storage Mode",
+            "modeHelp": "[TODO: Translate] Choose where .metadata.json sidecars and preview images are stored: next to each model file, or in a single centralized directory that mirrors your library structure. .civitai.info files always stay next to the model file.",
+            "modeOptions": {
+                "alongside": "[TODO: Translate] Alongside model files (default)",
+                "centralized": "[TODO: Translate] Centralized storage"
+            },
+            "path": "[TODO: Translate] Centralized Storage Path",
+            "pathHelp": "[TODO: Translate] Root directory for centralized sidecar storage. Leave empty to use the default location (<settings dir>/sidecars).",
+            "pathPlaceholder": "[TODO: Translate] Empty = <settings dir>/sidecars",
+            "management": "[TODO: Translate] Sidecar Migration",
+            "managementHelp": "[TODO: Translate] Move existing .metadata.json sidecars and preview images between alongside and centralized storage, matching the currently selected mode. Changing the mode does not move existing files automatically.",
+            "migrateButton": "[TODO: Translate] Migrate Sidecars Now",
+            "migratingButton": "[TODO: Translate] Migrating...",
+            "migrating": "[TODO: Translate] Migrating sidecars...",
+            "migrateSuccess": "[TODO: Translate] Sidecar migration completed successfully",
+            "migrateFailed": "[TODO: Translate] Sidecar migration failed: {message}",
+            "migrationDeferred": "[TODO: Translate] Existing sidecars were not moved. You can migrate them later from Settings → Library → Sidecar Storage.",
+            "confirmToCentralized": "[TODO: Translate] The storage mode changed, but existing .metadata.json sidecars and preview images are not moved automatically. Move them into the centralized storage directory now? You can also do this later with the \"Migrate Sidecars Now\" button.",
+            "confirmToAlongside": "[TODO: Translate] The storage mode changed, but existing .metadata.json sidecars and preview images are not moved automatically. Move them back next to their model files now? You can also do this later with the \"Migrate Sidecars Now\" button."
+        },
         "proxySettings": {
             "enableProxy": "啟用應用程式代理",
             "enableProxyHelp": "啟用此應用程式的自訂代理設定，將覆蓋系統代理設定",
@@ -1637,6 +1659,11 @@
             "titleRevert": "要還原原始檔案名稱嗎？",
             "revertButton": "還原原始檔案名稱"
         },
+        "sidecarMigrationConfirm": {
+            "titleToCentralized": "[TODO: Translate] Move sidecars to centralized storage?",
+            "titleToAlongside": "[TODO: Translate] Move sidecars back next to model files?",
+            "confirmButton": "[TODO: Translate] Migrate Now"
+        },
         "bulkAddTags": {
             "title": "新增標籤到多個模型",
             "description": "新增標籤到",