feat: add pass 3 text-line regression to deskew pipeline

After iterative projection (pass 1) and word-alignment (pass 2), a third
pass uses Tesseract word positions + linear regression per text line to
measure and correct residual rotation. This catches cases where passes 1-2
leave significant slope (e.g. 1.7° residual on heavily skewed scans).

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

klausur-service/backend/ocr_pipeline_api.py

@@ -488,12 +488,13 @@ async def auto_deskew(session_id: str):
 
     angle_iterative = two_pass_debug.get("pass1_angle", 0.0)
     angle_residual = two_pass_debug.get("pass2_angle", 0.0)
+    angle_textline = two_pass_debug.get("pass3_angle", 0.0)
 
     duration = time.time() - t0
 
-    method_used = "two_pass"
-    if abs(angle_residual) < 0.3:
-        method_used = "iterative"  # pass2 didn't contribute
+    method_used = "three_pass" if abs(angle_textline) >= 0.01 else (
+        "two_pass" if abs(angle_residual) >= 0.01 else "iterative"
+    )
 
     # Encode as PNG
     success, deskewed_png_buf = cv2.imencode(".png", deskewed_bgr)
@@ -515,6 +516,7 @@ async def auto_deskew(session_id: str):
         "angle_word_alignment": round(angle_wa, 3),
         "angle_iterative": round(angle_iterative, 3),
         "angle_residual": round(angle_residual, 3),
+        "angle_textline": round(angle_textline, 3),
         "angle_applied": round(angle_applied, 3),
         "method_used": method_used,
         "confidence": round(confidence, 2),
@@ -540,12 +542,14 @@ async def auto_deskew(session_id: str):
     logger.info(f"OCR Pipeline: deskew session {session_id}: "
                 f"hough={angle_hough:.2f} wa={angle_wa:.2f} "
                 f"iter={angle_iterative:.2f} residual={angle_residual:.2f} "
+                f"textline={angle_textline:.2f} "
                 f"-> {method_used} total={angle_applied:.2f}")
 
     await _append_pipeline_log(session_id, "deskew", {
         "angle_applied": round(angle_applied, 3),
         "angle_iterative": round(angle_iterative, 3),
         "angle_residual": round(angle_residual, 3),
+        "angle_textline": round(angle_textline, 3),
         "confidence": round(confidence, 2),
         "method": method_used,
     }, duration_ms=int(duration * 1000))