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.

py/services/model_file_service.py

@@ -8,6 +8,7 @@ from abc import ABC, abstractmethod
 
 from ..utils.utils import calculate_relative_path_for_model, remove_empty_dirs
 from ..utils.constants import AUTO_ORGANIZE_BATCH_SIZE, MODEL_FILE_EXTENSIONS
+from ..utils.sidecar_paths import is_centralized, resolve_centralized_dir_for_dir
 from ..services.settings_manager import get_settings_manager
 from ..services.model_lifecycle_service import _require_path_in_library_roots
 from ..services.pending_delete_service import PENDING_DELETE_DIR_NAME
@@ -631,6 +632,21 @@ class ModelMoveService:
 
             shutil.rmtree(absolute_path)
 
+            # Centralized mode: prune the folder's mirror subtree when it no
+            # longer holds any sidecar files (per-model deletes already
+            # removed their sidecars, so only empty directories are expected;
+            # a non-empty mirror keeps its orphan sidecars).
+            if is_centralized():
+                mirror_dir = resolve_centralized_dir_for_dir(absolute_path)
+                if mirror_dir and os.path.isdir(mirror_dir):
+                    for root, _dirs, files in os.walk(mirror_dir, topdown=False):
+                        if files:
+                            continue
+                        try:
+                            os.rmdir(root)
+                        except OSError:  # pragma: no cover - best-effort cleanup
+                            pass
+
             await self._forget_folder(relative_folder)
 
             return {