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/utils/file_lock.py

@@ -0,0 +1,146 @@
+"""Cross-process advisory locking for shared LoRA Manager state.
+
+Two LoRA Manager processes (the ComfyUI plugin and a standalone server, or two
+ComfyUI installs pointed at the same settings directory) can open the same cache
+database. SQLite serializes individual statements, but it cannot make a
+read-modify-write *sequence* atomic across processes: two full-table cache
+replacements can interleave so that one process's snapshot overwrites the
+other's.
+
+This module provides a small advisory file lock for those sequences. It is
+deliberately non-fatal: if locking is unavailable or the wait times out, callers
+keep working with SQLite's own ``busy_timeout`` as the fallback.
+"""
+
+from __future__ import annotations
+
+import logging
+import os
+import time
+
+logger = logging.getLogger(__name__)
+
+# How long to wait for another process to release the lock before giving up.
+DEFAULT_LOCK_TIMEOUT_SECONDS = 30.0
+_POLL_INTERVAL_SECONDS = 0.05
+
+# Windows byte-range locks; fcntl.flock on POSIX.
+try:  # pragma: no cover - platform dependent
+    import fcntl
+except ImportError:  # pragma: no cover - Windows
+    fcntl = None  # type: ignore[assignment]
+
+try:  # pragma: no cover - Windows only
+    import msvcrt
+except ImportError:  # pragma: no cover - POSIX
+    msvcrt = None  # type: ignore[assignment]
+
+
+class FileLockUnavailable(RuntimeError):
+    """Raised when the lock could not be acquired within the timeout."""
+
+
+def lock_path_for(db_path: str) -> str:
+    """Return the sibling lock file path used for *db_path*."""
+    absolute = os.path.abspath(db_path)
+    directory = os.path.dirname(absolute)
+    if not directory:
+        raise ValueError(f"Cannot derive a lock directory from {db_path!r}")
+    return os.path.join(directory, f".{os.path.basename(absolute)}.lock")
+
+
+class CrossProcessLock:
+    """A best-effort advisory lock backed by a lock file.
+
+    The lock file is a sibling of the guarded resource and is never deleted:
+    unlinking it would let a second process create a fresh inode and lock that
+    instead, defeating mutual exclusion.
+    """
+
+    def __init__(self, path: str, timeout: float = DEFAULT_LOCK_TIMEOUT_SECONDS):
+        self.path = path
+        self.timeout = timeout
+        self._handle = None
+
+    def acquire(self) -> bool:
+        """Try to take the lock, waiting up to ``timeout`` seconds.
+
+        Returns:
+            True when the lock is held (including when another lock is already
+            held by *this* process — the calls are not reentrant, so callers must
+            not nest them). False when locking is unsupported or timed out; the
+            caller should proceed and rely on the SQLite busy timeout instead.
+        """
+        if fcntl is None and msvcrt is None:  # pragma: no cover - exotic platform
+            return False
+
+        os.makedirs(os.path.dirname(self.path), exist_ok=True)
+        try:
+            handle = open(self.path, "a+b")
+        except OSError as exc:
+            logger.debug("Could not open lock file %s: %s", self.path, exc)
+            return False
+
+        deadline = time.monotonic() + max(0.0, self.timeout)
+        while True:
+            if self._try_lock(handle):
+                self._handle = handle
+                return True
+            if time.monotonic() >= deadline:
+                handle.close()
+                return False
+            time.sleep(_POLL_INTERVAL_SECONDS)
+
+    def release(self) -> None:
+        """Release the lock if held. Safe to call more than once."""
+        handle = self._handle
+        if handle is None:
+            return
+        self._handle = None
+        try:
+            self._unlock(handle)
+        except OSError as exc:  # pragma: no cover - defensive
+            logger.debug("Failed to release lock %s: %s", self.path, exc)
+        finally:
+            try:
+                handle.close()
+            except OSError:  # pragma: no cover - defensive
+                pass
+
+    def __enter__(self) -> "CrossProcessLock":
+        self.acquire()
+        return self
+
+    def __exit__(self, *_exc_info: object) -> None:
+        self.release()
+
+    # -- platform primitives -------------------------------------------------
+
+    def _try_lock(self, handle) -> bool:
+        if fcntl is not None:
+            try:
+                fcntl.flock(handle.fileno(), fcntl.LOCK_EX | fcntl.LOCK_NB)
+                return True
+            except OSError:
+                return False
+        if msvcrt is not None:  # pragma: no cover - Windows
+            try:
+                handle.seek(0)
+                msvcrt.locking(handle.fileno(), msvcrt.LK_NBLCK, 1)
+                return True
+            except OSError:
+                return False
+        return False
+
+    def _unlock(self, handle) -> None:
+        if fcntl is not None:
+            fcntl.flock(handle.fileno(), fcntl.LOCK_UN)
+            return
+        if msvcrt is not None:  # pragma: no cover - Windows
+            handle.seek(0)
+            msvcrt.locking(handle.fileno(), msvcrt.LK_UNLCK, 1)
+
+
+def exclusive_lock(db_path: str, timeout: float = DEFAULT_LOCK_TIMEOUT_SECONDS):
+    """Return a :class:`CrossProcessLock` for the database at *db_path*."""
+    return CrossProcessLock(lock_path_for(db_path), timeout=timeout)