feat: iterative projection-profile deskew (2-phase variance optimization)

Adds deskew_image_iterative() as 3rd deskew method that directly optimizes
for projection-profile sharpness instead of proxy signals (Hough/word alignment).
Coarse sweep on horizontal profile, fine sweep on vertical profile.

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

klausur-service/backend/ocr_pipeline_api.py

@@ -52,6 +52,7 @@ from cv_vocab_pipeline import (
     create_ocr_image,
     deskew_image,
     deskew_image_by_word_alignment,
+    deskew_image_iterative,
     detect_column_geometry,
     detect_document_type,
     detect_row_geometry,
@@ -485,10 +486,23 @@ async def auto_deskew(session_id: str):
         logger.warning(f"Word alignment deskew failed: {e}")
         deskewed_wa_bytes, angle_wa = orig_bytes, 0.0
 
+    # Method 3: Iterative Projection-Profile
+    angle_iterative = 0.0
+    iterative_debug = {}
+    try:
+        deskewed_iter, angle_iterative, iterative_debug = deskew_image_iterative(img_bgr.copy())
+    except Exception as e:
+        logger.warning(f"Iterative deskew failed: {e}")
+        deskewed_iter = img_bgr
+
     duration = time.time() - t0
 
-    # Pick best method
-    if abs(angle_wa) >= abs(angle_hough) or abs(angle_hough) < 0.1:
+    # Pick best method — prefer iterative when it found a non-zero angle
+    if abs(angle_iterative) >= 0.05:
+        method_used = "iterative"
+        angle_applied = angle_iterative
+        deskewed_bgr = deskewed_iter
+    elif abs(angle_wa) >= abs(angle_hough) or abs(angle_hough) < 0.1:
         method_used = "word_alignment"
         angle_applied = angle_wa
         wa_array = np.frombuffer(deskewed_wa_bytes, dtype=np.uint8)
@@ -520,6 +534,7 @@ async def auto_deskew(session_id: str):
     deskew_result = {
         "angle_hough": round(angle_hough, 3),
         "angle_word_alignment": round(angle_wa, 3),
+        "angle_iterative": round(angle_iterative, 3),
         "angle_applied": round(angle_applied, 3),
         "method_used": method_used,
         "confidence": round(confidence, 2),
@@ -542,7 +557,8 @@ async def auto_deskew(session_id: str):
     await update_session_db(session_id, **db_update)
 
     logger.info(f"OCR Pipeline: deskew session {session_id}: "
-                f"hough={angle_hough:.2f} wa={angle_wa:.2f} -> {method_used} {angle_applied:.2f}")
+                f"hough={angle_hough:.2f} wa={angle_wa:.2f} iter={angle_iterative:.2f} "
+                f"-> {method_used} {angle_applied:.2f}")
 
     await _append_pipeline_log(session_id, "deskew", {
         "angle_applied": round(angle_applied, 3),