feat: OCR scans with tesseract, drop docling from the default path.

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.go

@@ -0,0 +1,162 @@
+// Package ocr runs Tesseract (eng+deu) on images and scanned PDFs.
+//
+// Default engine is the tesseract CLI, not gosseract CGO: Ladybug CGO stays
+// Zig-only (D21). Same engine, no gocv. OCR_ENGINE=paddle selects paddleocr
+// when that binary is on PATH (compose profile ocr-paddle).
+package ocr
+
+import (
+	"fmt"
+	"image"
+	"image/color"
+	"image/png"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+)
+
+const TessLang = "eng+deu"
+
+func ImageFile(path string) (string, error) {
+	engine := os.Getenv("OCR_ENGINE")
+	if engine == "paddle" {
+		return runPaddle(path)
+	}
+	return runTesseract(path)
+}
+
+func PDFFile(path string) (string, error) {
+	text, err := pdfToText(path)
+	if err == nil && strings.TrimSpace(text) != "" {
+		return strings.TrimSpace(text), nil
+	}
+	ocr, oerr := pdfPages(path)
+	if oerr != nil {
+		if err != nil {
+			return "", err
+		}
+		return "", oerr
+	}
+	if strings.TrimSpace(ocr) != "" {
+		return strings.TrimSpace(ocr), nil
+	}
+	if text != "" {
+		return strings.TrimSpace(text), nil
+	}
+	return "", fmt.Errorf("pdf has no text layer (ocr unavailable)")
+}
+
+func pdfToText(path string) (string, error) {
+	cmd := exec.Command("pdftotext", "-layout", path, "-")
+	out, err := cmd.Output()
+	if err != nil {
+		return "", err
+	}
+	return string(out), nil
+}
+
+func pdfPages(path string) (string, error) {
+	dir, err := os.MkdirTemp("", "2dph-ocr-")
+	if err != nil {
+		return "", err
+	}
+	defer os.RemoveAll(dir)
+	prefix := filepath.Join(dir, "page")
+	cmd := exec.Command("pdftoppm", "-png", "-r", "200", path, prefix)
+	if err := cmd.Run(); err != nil {
+		return "", err
+	}
+	matches, err := filepath.Glob(prefix + "*.png")
+	if err != nil {
+		return "", err
+	}
+	var parts []string
+	for _, img := range matches {
+		t, err := ImageFile(img)
+		if err != nil {
+			continue
+		}
+		if s := strings.TrimSpace(t); s != "" {
+			parts = append(parts, s)
+		}
+	}
+	return strings.Join(parts, "\n\n"), nil
+}
+
+func runTesseract(path string) (string, error) {
+	pre, err := preprocessFile(path)
+	if err != nil {
+		pre = path
+	} else {
+		defer os.Remove(pre)
+	}
+	cmd := exec.Command("tesseract", pre, "stdout", "-l", TessLang, "--psm", "6")
+	out, err := cmd.Output()
+	if err != nil {
+		return "", err
+	}
+	return strings.TrimSpace(string(out)), nil
+}
+
+func runPaddle(path string) (string, error) {
+	cmd := exec.Command("paddleocr", "ocr", "-i", path)
+	out, err := cmd.Output()
+	if err != nil {
+		return "", err
+	}
+	return strings.TrimSpace(string(out)), nil
+}
+
+func preprocessFile(path string) (string, error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return "", err
+	}
+	defer f.Close()
+	img, err := png.Decode(f)
+	if err != nil {
+		return "", err
+	}
+	out := filepath.Join(os.TempDir(), filepath.Base(path)+".gray.png")
+	w, err := os.Create(out)
+	if err != nil {
+		return "", err
+	}
+	defer w.Close()
+	if err := png.Encode(w, GrayContrast(img)); err != nil {
+		os.Remove(out)
+		return "", err
+	}
+	return out, nil
+}
+
+// GrayContrast is a stdlib preprocess (no gocv): grayscale + stretch.
+func GrayContrast(src image.Image) image.Image {
+	b := src.Bounds()
+	dst := image.NewGray(b)
+	var minL, maxL uint8 = 255, 0
+	for y := b.Min.Y; y < b.Max.Y; y++ {
+		for x := b.Min.X; x < b.Max.X; x++ {
+			g := color.GrayModel.Convert(src.At(x, y)).(color.Gray)
+			if g.Y < minL {
+				minL = g.Y
+			}
+			if g.Y > maxL {
+				maxL = g.Y
+			}
+		}
+	}
+	span := int(maxL) - int(minL)
+	if span < 1 {
+		span = 1
+	}
+	for y := b.Min.Y; y < b.Max.Y; y++ {
+		for x := b.Min.X; x < b.Max.X; x++ {
+			g := color.GrayModel.Convert(src.At(x, y)).(color.Gray)
+			v := uint8((int(g.Y) - int(minL)) * 255 / span)
+			dst.SetGray(x, y, color.Gray{Y: v})
+		}
+	}
+	return dst
+}