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/pending_delete_service.py

@@ -38,7 +38,7 @@ from typing import (
 )
 
 from ..utils.constants import PREVIEW_EXTENSIONS
-from ..utils.sidecar_paths import get_metadata_path
+from ..utils.sidecar_paths import get_metadata_path, get_sidecar_dir
 from ..utils import settings_paths
 
 logger = logging.getLogger(__name__)
@@ -670,16 +670,22 @@ class PendingDeleteService:
         """Enumerate existing artifacts exactly like delete_model_artifacts."""
         main_extension = ".safetensors" if main_extension is None else main_extension
         main_file = f"{file_name}{main_extension}" if main_extension else file_name
-        patterns = [
-            main_file,
-            os.path.basename(get_metadata_path(os.path.join(target_dir, main_file))),
-        ]
+        model_path = os.path.join(target_dir, main_file)
+
+        artifacts: List[str] = []
+        main_path = os.path.abspath(model_path)
+        if os.path.exists(main_path):
+            artifacts.append(main_path)
+
+        # Sidecars/previews live in the sidecar dir (the model's own dir in
+        # alongside mode, the centralized mirror otherwise).
+        sidecar_dir = get_sidecar_dir(model_path)
+        patterns = [os.path.basename(get_metadata_path(model_path))]
         for ext in PREVIEW_EXTENSIONS:
             patterns.append(f"{file_name}{ext}")
 
-        artifacts: List[str] = []
         for pattern in patterns:
-            path = os.path.abspath(os.path.join(target_dir, pattern))
+            path = os.path.abspath(os.path.join(sidecar_dir, pattern))
             if os.path.exists(path):
                 artifacts.append(path)
         return artifacts
@@ -698,7 +704,9 @@ class PendingDeleteService:
         """
         for original_path in artifacts:
             staged_path = os.path.join(batch_dir, os.path.basename(original_path))
-            os.rename(original_path, staged_path)
+            # EXDEV-tolerant: centralized sidecars may live on a different
+            # filesystem than the staging batch dir under the model root.
+            self._restore_file(original_path, staged_path)
             staged_pairs.append(
                 {
                     "staged": os.path.abspath(staged_path),
@@ -740,13 +748,14 @@ class PendingDeleteService:
         return staged_pairs
 
     def _restore_file(self, staged_path: str, original_path: str) -> None:
-        """Restore a staged file to its original path, tolerating EXDEV.
+        """Move a file between staging and library paths, tolerating EXDEV.
 
         ``os.rename`` is atomic and preferred (model staging and most recipe
         restores are same-volume). Recipe staging copies into the settings-dir
-        staging parent, which may live on a DIFFERENT filesystem than the
-        recipes dir; rename then raises EXDEV. Fall back to ``shutil.copy2`` +
-        ``os.remove`` so the bytes are restored and the staged copy removed.
+        staging parent, and centralized sidecars live under the configured
+        sidecar root; both may live on a DIFFERENT filesystem than the target
+        dir, so rename can raise EXDEV. Fall back to ``shutil.copy2`` +
+        ``os.remove`` so the bytes are moved and the source copy removed.
         """
         try:
             os.rename(staged_path, original_path)