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/checkpoint_scanner.py

@@ -12,6 +12,7 @@ from typing import Any, Dict, List, Optional
 from ..utils.models import CheckpointMetadata
 from ..utils.file_utils import find_preview_file, normalize_path, calculate_autov3
 from ..utils.metadata_manager import MetadataManager
+from ..utils.sidecar_paths import get_preview_dir, is_centralized
 from ..config import config
 from .model_scanner import ModelScanner, _is_excluded_dir
 from .model_hash_index import ModelHashIndex
@@ -61,10 +62,9 @@ class CheckpointScanner(ModelScanner):
                 return None
 
             base_name = os.path.splitext(os.path.basename(file_path))[0]
-            dir_path = os.path.dirname(file_path)
 
             # Find preview image
-            preview_url = find_preview_file(base_name, dir_path)
+            preview_url = find_preview_file(base_name, get_preview_dir(file_path))
 
             # AutoV3 reads only the safetensors header, so it is cheap even for
             # large checkpoints; record the checked state at creation time ("" =
@@ -322,6 +322,11 @@ class CheckpointScanner(ModelScanner):
 
     async def _find_pending_models_from_filesystem(self) -> List[Dict[str, Any]]:
         """Scan filesystem for checkpoint metadata files with pending hash status."""
+        # Centralized mode stores sidecars in the mirror tree, not next to the
+        # models; walk the mirror instead of the model folders.
+        if is_centralized():
+            return self._find_pending_models_in_sidecar_mirror()
+
         pending_models = []
 
         for root_path in self.get_model_roots():