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.

AGENTS.md

@@ -266,6 +266,13 @@ If a cross-layer issue ever needs a live server, the sandboxed helpers live in
   them during migration/import. Never write, modify, or delete them, and never
   propose doing so as a fix — LoRA Manager's own metadata lives in the
   `.metadata.json` sidecar it owns.
+- **Sidecar/preview path derivation must go through `py/utils/sidecar_paths.py`**
+  helpers (never inline `splitext + ".metadata.json"`): the centralized storage
+  mode (`sidecar_storage_mode` / `sidecar_storage_path` settings) relocates
+  `.metadata.json` files and preview images under a mirror tree, so any
+  hand-built path is wrong in that mode. `.civitai.info` stays co-located with
+  the model file in both modes. The new settings keys live only in
+  `DEFAULT_SETTINGS` — `settings.json.example` stays minimal (see below).
 - **`settings.json.example` must stay minimal**: only `use_portable_settings`,
   `civitai_api_key`, and the four core `folder_paths` keys (`loras`,
   `checkpoints`, `unet`, `embeddings`). Do NOT add optional/default keys