fix(sidecars): carry the identity map through a sidecar root move

A model root that moved earlier is mirrored under a pinned name derived
from its old path. If anything resolved against the new sidecar path before
the relocation ran, the destination got a map naming the mirror after the
*current* path. migrate_root's keep-newer transfer then dropped the source
map, so the moved metadata stayed orphaned under the pinned component while
reads followed the new name and rebuilt defaults — losing favorites, notes
and tags a second time.

The identity map is now relocated by relocate_root_map() instead of the
generic transfer: entries recorded under the old sidecar root win for the
roots they describe, destination-only entries are preserved, and the cache
is dropped so the next resolution reloads the merged map. A merge that
cannot be written is reported as a migration error rather than silently
stranding the moved metadata.

Reported by the Codex review on #1131.

docs/metadata-json-schema.md

@@ -32,7 +32,7 @@ In centralized mode, sidecars and previews mirror each model root's directory st
 - If an identity cannot be re-anchored unambiguously (e.g. two same-named candidate roots), nothing is guessed: the mirror stays on disk untouched and surfaces as an orphan in **Doctor → Centralized Sidecars** (and in the log). Restoring the original root path re-links it automatically.
 - `.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). The migration covers excluded (hidden) models too, so un-excluding one later never strands its sidecar in the old layout. The result payload includes a `sidecar_root` field with the resolved centralized root, and the settings UI shows the outcome counters plus an "Open Folder" shortcut.
-- Changing `sidecar_storage_path` while centralized likewise needs a root relocation: `{"direction": "relocate_root", "old_root": "<previous path>"}` moves the whole mirror tree to the new root (the settings UI offers this automatically).
+- Changing `sidecar_storage_path` while centralized likewise needs a root relocation: `{"direction": "relocate_root", "old_root": "<previous path>"}` moves the whole mirror tree to the new root (the settings UI offers this automatically). The identity map travels with the tree, and its entries win over any map the destination acquired beforehand — so a mirror that was re-anchored earlier keeps its name even if something resolved against the new path before the relocation ran.
 - The settings UI always shows the resolved effective storage root (via the `sidecar_storage_root*` fields in `GET /api/lm/settings`), with `POST /api/lm/sidecars/open-location` opening it in the file manager. When the resolved root lies inside the plugin installation folder (portable settings mode), the UI warns: reinstalling or clean-updating the plugin would delete the sidecars, so an explicit path outside the installation folder is recommended. The repo `.gitignore` excludes the portable-mode default (`/sidecars/`).
 - All sidecar/preview path derivation goes through the helpers in `py/utils/sidecar_paths.py`; never construct paths inline. In the default `alongside` mode these helpers do no extra I/O at all — the identity map is only loaded and reconciled when centralized storage is actually in use.