test: skip watermark integration when darktable export fails

Add darktable style probe and pytest integration/slow markers so CI and
machines without a working darktable-cli setup skip the full workflow test.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/integration_helpers.py

@@ -0,0 +1,53 @@
+"""Shared helpers for integration tests."""
+
+from __future__ import annotations
+
+import shutil
+import subprocess
+import tempfile
+from pathlib import Path
+
+from tests.conftest import make_png
+
+
+def has_darktable_style(style_name: str, config_dir: Path | None = None) -> bool:
+    """Return True if darktable-cli can apply ``style_name`` to a tiny PNG."""
+    if not shutil.which("darktable-cli"):
+        return False
+
+    config = config_dir or Path.home() / ".config" / "darktable"
+    styles_dir = config / "styles"
+    if not styles_dir.is_dir():
+        return False
+    if not any(p.stem == style_name for p in styles_dir.glob("*.dtstyle")):
+        return False
+
+    with tempfile.TemporaryDirectory() as tmp:
+        tmp_path = Path(tmp)
+        src = tmp_path / "probe.png"
+        out_dir = tmp_path / "out"
+        out_dir.mkdir()
+        make_png(src)
+        cmd = [
+            "darktable-cli",
+            str(src),
+            str(out_dir),
+            "--style",
+            style_name,
+            "--out-ext",
+            "png",
+            "--core",
+            "--configdir",
+            str(config),
+            "--style-overwrite",
+        ]
+        result = subprocess.run(cmd, capture_output=True, text=True)
+        if result.returncode != 0:
+            return False
+        return any(out_dir.iterdir())
+
+
+def has_workflow_tools() -> bool:
+    """True when rembg, gmic, ImageMagick, and darktable-cli are on PATH."""
+    has_magick = bool(shutil.which("magick") or shutil.which("convert"))
+    return all(shutil.which(name) for name in ("rembg", "gmic", "darktable-cli")) and has_magick