feat: OCR scans with tesseract, drop docling from the default path. (#33)

pdftotext still wins on born-digital PDFs. Empty text layers go through
pdftoppm + tesseract eng+deu. Optional OCR_ENGINE=paddle. Gitea #6.

internal/ocr/ocr_test.go

@@ -0,0 +1,54 @@
+package ocr
+
+import (
+	"image"
+	"image/color"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestGrayContrastStretches(t *testing.T) {
+	img := image.NewGray(image.Rect(0, 0, 2, 2))
+	img.SetGray(0, 0, color.Gray{Y: 64})
+	img.SetGray(0, 1, color.Gray{Y: 64})
+	img.SetGray(1, 0, color.Gray{Y: 64})
+	img.SetGray(1, 1, color.Gray{Y: 192})
+	out := GrayContrast(img).(*image.Gray)
+	if out.GrayAt(0, 0).Y != 0 {
+		t.Fatalf("min should map to 0, got %d", out.GrayAt(0, 0).Y)
+	}
+	if out.GrayAt(1, 1).Y != 255 {
+		t.Fatalf("max should map to 255, got %d", out.GrayAt(1, 1).Y)
+	}
+}
+
+func TestHelloPNGFixtureOCR(t *testing.T) {
+	if _, err := exec.LookPath("tesseract"); err != nil {
+		t.Skip("tesseract not installed")
+	}
+	path := filepath.Join("testdata", "hello.png")
+	got, err := ImageFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	up := strings.ToUpper(got)
+	if !strings.Contains(up, "HELLO") {
+		t.Fatalf("ocr %q missing HELLO", got)
+	}
+}
+
+func TestPaddleEngineUsesPaddleocrBinary(t *testing.T) {
+	t.Setenv("OCR_ENGINE", "paddle")
+	_, err := ImageFile(filepath.Join("testdata", "hello.png"))
+	if _, look := exec.LookPath("paddleocr"); look != nil {
+		if err == nil {
+			t.Fatal("expected error when paddleocr is missing")
+		}
+		return
+	}
+	if err != nil {
+		t.Fatal(err)
+	}
+}