metadata writer: add guard for exiftools installed on system level

src/photools3/metadata_writer.py

@@ -1,4 +1,5 @@
 import logging
+import shutil
 from pathlib import Path
 from typing import Annotated, Any
 
@@ -9,6 +10,13 @@ from photools3.settings import settings
 
 log = logging.getLogger(__name__)
 
+_EXIFTOOL_MISSING_MSG = (
+    "exiftool binary not found in PATH. "
+    "Install the system package: 'perl-image-exiftool' (Arch) or 'libimage-exiftool-perl' (Debian/Ubuntu)."
+)
+if shutil.which("exiftool") is None:
+    raise FileNotFoundError(_EXIFTOOL_MISSING_MSG)
+
 # File extensions that should use XMP sidecars
 SIDECAR_EXTENSIONS = {".dng", ".nef", ".cr2", ".cr3", ".arw", ".raf", ".orf", ".rw2"}