perf(rename): make bulk filename-template apply O(n) instead of O(n^2)

Applying a filename template to a large library re-did O(library) work for
every renamed file: a full natsort resort plus whole-table SQLite rewrite and
download-history resync after each rename, and a full scan plus resort of the
entire recipe collection per renamed LoRA. On a 20k-model library with 300k
recipes on a HDD this pushed "Apply to Library" into multi-day runs.

- ModelScanner.defer_cache_persist(): bulk loops update the in-memory entry
  and indexes only; resort + persist + download-history sync run once at
  context exit, forced even on cancellation/error since files are already
  renamed on disk. Single-rename callers keep immediate per-call behavior.
- RecipeScanner.build_lora_hash_index(): one-shot hash -> recipes index so
  per-file lookups are O(1); update_lora_filename_by_hash gains hash_index /
  defer_maintenance params, with a single finalize_bulk_filename_updates()
  resort at the end of a bulk session.
- ModelLifecycleService.bulk_rename_session() / BulkRenameContext wire the
  deferred path through rename_model (hash index built lazily on first
  recipe-touching rename).
- Blocking os.rename sequence offloaded via asyncio.to_thread so one file's
  HDD I/O no longer stalls the event loop (no cross-file parallelism).
- Skip logic, per-batch WebSocket progress, cancellation, and result
  counters unchanged.

tests/conftest.py

@@ -3,9 +3,10 @@ import importlib.util
 import inspect
 import sys
 import types
+from contextlib import asynccontextmanager
 from dataclasses import dataclass, field
 from pathlib import Path
-from typing import Any, Dict, List, Optional, Sequence
+from typing import Any, AsyncIterator, Dict, List, Optional, Sequence
 from unittest import mock
 
 import pytest
@@ -177,6 +178,10 @@ class MockScanner:
     def reset_cancellation(self) -> None:
         self._cancelled = False
 
+    @asynccontextmanager
+    async def defer_cache_persist(self) -> AsyncIterator[None]:
+        yield None
+
     async def get_cached_data(self, force_refresh: bool = False):
         return self._cache