feat: automatische Orientierungserkennung fuer umgedrehte Scans

Tesseract OSD erkennt 0/90/180/270° Rotation und korrigiert
automatisch vor dem Deskew. Loest das Problem mit Buchscannern,
bei denen jede 2. Seite auf dem Kopf steht.

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

klausur-service/backend/cv_vocab_pipeline.py

@@ -224,6 +224,54 @@ def render_image_high_res(image_data: bytes) -> np.ndarray:
     return img_bgr
 
 
+# =============================================================================
+# Stage 1b: Orientation Detection (0°/90°/180°/270°)
+# =============================================================================
+
+def detect_and_fix_orientation(img_bgr: np.ndarray) -> Tuple[np.ndarray, int]:
+    """Detect page orientation via Tesseract OSD and rotate if needed.
+
+    Handles upside-down scans (180°) common with book scanners where
+    every other page is flipped due to the scanner hinge.
+
+    Returns:
+        (corrected_image, rotation_degrees)  — rotation is 0, 90, 180, or 270.
+    """
+    if pytesseract is None:
+        return img_bgr, 0
+
+    try:
+        # Tesseract OSD needs a grayscale or RGB image
+        gray = cv2.cvtColor(img_bgr, cv2.COLOR_BGR2GRAY)
+        pil_img = Image.fromarray(gray)
+
+        osd = pytesseract.image_to_osd(pil_img, output_type=pytesseract.Output.DICT)
+        rotate = osd.get("rotate", 0)
+        confidence = osd.get("orientation_conf", 0.0)
+
+        logger.info(f"OSD: orientation={rotate}° confidence={confidence:.1f}")
+
+        if rotate == 0 or confidence < 1.0:
+            return img_bgr, 0
+
+        # Apply rotation
+        if rotate == 180:
+            corrected = cv2.rotate(img_bgr, cv2.ROTATE_180)
+        elif rotate == 90:
+            corrected = cv2.rotate(img_bgr, cv2.ROTATE_90_COUNTERCLOCKWISE)
+        elif rotate == 270:
+            corrected = cv2.rotate(img_bgr, cv2.ROTATE_90_CLOCKWISE)
+        else:
+            return img_bgr, 0
+
+        logger.info(f"OSD: rotated {rotate}° to fix orientation")
+        return corrected, rotate
+
+    except Exception as e:
+        logger.warning(f"OSD orientation detection failed: {e}")
+        return img_bgr, 0
+
+
 # =============================================================================
 # Stage 2: Deskew (Rotation Correction)
 # =============================================================================