fix(cache): make shared cache state survive a second instance

Installing a second LoRA Manager instance (standalone or a second
ComfyUI install) that shares the settings directory puts two processes
on the same cache databases. Three things made that unsafe.

- The updater preserved cache/ and model_cache/ but not a legacy
  recipe_cache/ directory, so a portable install predating the cache/
  move lost its recipe database on a git-based update. Add it to
  _PRESERVE_DIRS and to .gitignore.
- Cache connections used the sqlite3 default 5s timeout, which a
  scanning instance can exceed, turning a concurrent write into
  "database is locked". Route every shared cache connection through
  connect_cache_db(), which raises the timeout to 30s and sets
  busy_timeout + synchronous=NORMAL to match the existing WAL mode.
  App-private databases (download queue, update history) are unchanged.
- A full-table cache replace is a read-modify-write that SQLite cannot
  make atomic across processes, so two instances could interleave and
  one snapshot could overwrite the other. Guard the recipe and model
  save_cache paths with a cross-process advisory lock (flock on POSIX,
  msvcrt on Windows). Locking is best-effort: if it is unavailable the
  call proceeds and the SQLite busy timeout is the fallback.

The lock file is a hidden sibling of the database and is deliberately
never unlinked, so a second process cannot lock a fresh inode.

py/services/recipe_scanner.py

@@ -2729,6 +2729,10 @@ class RecipeScanner:
                     try:
                         # Invalidate persistent cache so the sync path does a
                         # full directory scan instead of reconciling stale data.
+                        # This is the deliberate escape hatch from the
+                        # all-missing prune guard: an explicit user rebuild is
+                        # allowed to clear the stored cache, while an implicit
+                        # startup scan is not.
                         if self._persistent_cache:
                             self._persistent_cache.save_cache([], {})
                         self._json_path_map = {}