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.

docs/metadata-json-schema.md

@@ -11,6 +11,28 @@ This document defines the complete schema for `.metadata.json` files used by Lor
 
 ---
 
+## Storage Location (Alongside vs Centralized)
+
+By default, `.metadata.json` sidecars and preview images live **alongside** their model files. An optional centralized mode stores them under a single root directory instead. Two settings control this (Settings → Library → Sidecar Storage):
+
+| Setting | Values | Default |
+|---------|--------|---------|
+| `sidecar_storage_mode` | `"alongside"` \| `"centralized"` | `"alongside"` |
+| `sidecar_storage_path` | Absolute path string; empty = `<settings dir>/sidecars` | `""` |
+
+In centralized mode, sidecars and previews mirror the library-relative directory structure:
+
+```
+<sidecar_root>/<library>/<root_basename>/<rel_dir>/<name>.metadata.json
+```
+
+- `<library>` is the active library name, `<root_basename>` the basename of the model root containing the file, and `<rel_dir>` the model's directory relative to that root. Each component is sanitized to filesystem-safe characters.
+- `.civitai.info` files always stay next to the model file, in both modes.
+- Changing the mode does **not** move existing files automatically — run the migration (`POST /api/lm/sidecars/migrate` with `{"direction": "to_centralized" | "to_alongside"}`, or the "Migrate Sidecars Now" button in settings).
+- All sidecar/preview path derivation goes through the helpers in `py/utils/sidecar_paths.py`; never construct paths inline.
+
+---
+
 ## Base Fields (All Model Types)
 
 These fields are present in all model metadata files.