fix: Preflight-Check und Exit-Code in --once Modus (v0.2.1)

- #1: check_preflight() prüft beim Start tesseract + gs, wirft
  PreflightError. CLI endet mit Exit 2 statt grün zu bleiben.
- #2: run_once() gibt Anzahl fehlgeschlagener PDFs zurück, CLI
  endet mit Exit 1 wenn mindestens eine Datei scheiterte.
- pytest-Suite mit 11 Tests für beide Szenarien
- ocrmypdf-Import lazy in processor.py (Tests ohne ocrmypdf möglich)

Closes #1, #2

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/test_preflight.py

@@ -0,0 +1,75 @@
+"""Tests für Issue #1: Preflight-Check bei fehlendem Tesseract."""
+from __future__ import annotations
+
+import sys
+from unittest.mock import patch
+
+import pytest
+
+from pdf_ocr_hotfolder.service import (
+    HotfolderService,
+    PreflightError,
+    check_preflight,
+)
+
+
+def test_preflight_passes_when_all_binaries_present() -> None:
+    """Wenn tesseract + gs im PATH sind, darf kein Fehler fliegen."""
+    with patch("pdf_ocr_hotfolder.service.shutil.which", return_value="/usr/bin/fake"):
+        check_preflight()  # darf nicht werfen
+
+
+def test_preflight_fails_when_tesseract_missing() -> None:
+    """Fehlendes tesseract → PreflightError mit passender Meldung."""
+    def fake_which(name: str) -> str | None:
+        return None if name == "tesseract" else "/usr/bin/fake"
+
+    with patch("pdf_ocr_hotfolder.service.shutil.which", side_effect=fake_which):
+        with pytest.raises(PreflightError, match="tesseract"):
+            check_preflight()
+
+
+def test_preflight_fails_when_ghostscript_missing() -> None:
+    def fake_which(name: str) -> str | None:
+        return None if name == "gs" else "/usr/bin/fake"
+
+    with patch("pdf_ocr_hotfolder.service.shutil.which", side_effect=fake_which):
+        with pytest.raises(PreflightError, match="gs"):
+            check_preflight()
+
+
+def test_preflight_lists_all_missing_binaries() -> None:
+    """Bei mehreren fehlenden Binaries werden alle genannt."""
+    with patch("pdf_ocr_hotfolder.service.shutil.which", return_value=None):
+        with pytest.raises(PreflightError) as exc_info:
+            check_preflight()
+        msg = str(exc_info.value)
+        assert "tesseract" in msg
+        assert "gs" in msg
+
+
+def test_run_once_raises_preflight_error(tmp_config) -> None:
+    """HotfolderService.run_once() wirft PreflightError, wenn tesseract fehlt."""
+    service = HotfolderService(tmp_config)
+    try:
+        with patch("pdf_ocr_hotfolder.service.shutil.which", return_value=None):
+            with pytest.raises(PreflightError):
+                service.run_once()
+    finally:
+        service._executor.shutdown(wait=False)
+
+
+def test_main_returns_2_on_preflight_error(tmp_config, tmp_path, monkeypatch) -> None:
+    """CLI liefert Exit-Code 2 bei Preflight-Fehler (Issue #1 Szenario)."""
+    cfg_file = tmp_path / "cfg.toml"
+    cfg_file.write_text(f"""
+[paths]
+incoming = "{tmp_config.paths.incoming}"
+outgoing = "{tmp_config.paths.outgoing}"
+working  = "{tmp_config.paths.working}"
+error    = "{tmp_config.paths.error}"
+""")
+    monkeypatch.setattr(sys, "argv", ["pdf-ocr-hotfolder", "--config", str(cfg_file), "--once"])
+    with patch("pdf_ocr_hotfolder.service.shutil.which", return_value=None):
+        from pdf_ocr_hotfolder.__main__ import main
+        assert main() == 2