fix: add debug image saving gated on LOG_LEVEL=debug (refs #113)

Save original, adaptive, and Otsu preprocessed images to
/tmp/vin-debug/{timestamp}/ when LOG_LEVEL is set to debug.
No images saved at info level. Volume mount added for access.

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

ocr/app/extractors/vin_extractor.py

@@ -1,8 +1,10 @@
 """VIN-specific OCR extractor with preprocessing and validation."""
 import io
 import logging
+import os
 import time
 from dataclasses import dataclass, field
+from datetime import datetime
 from typing import Optional
 
 import magic
@@ -57,9 +59,31 @@ class VinExtractor(BaseExtractor):
     # VIN character whitelist for Tesseract
     VIN_WHITELIST = "ABCDEFGHJKLMNPRSTUVWXYZ0123456789"
 
+    # Fixed debug output directory (inside container)
+    DEBUG_DIR = "/tmp/vin-debug"
+
     def __init__(self) -> None:
         """Initialize VIN extractor."""
         pytesseract.pytesseract.tesseract_cmd = settings.tesseract_cmd
+        self._debug = settings.log_level.upper() == "DEBUG"
+
+    def _save_debug_image(self, session_dir: str, name: str, data: bytes) -> None:
+        """Save image bytes to the debug session directory when LOG_LEVEL=debug."""
+        if not self._debug:
+            return
+        path = os.path.join(session_dir, name)
+        with open(path, "wb") as f:
+            f.write(data)
+        logger.debug("Saved debug image: %s (%d bytes)", name, len(data))
+
+    def _create_debug_session(self) -> Optional[str]:
+        """Create a timestamped debug directory. Returns path or None."""
+        if not self._debug:
+            return None
+        ts = datetime.now().strftime("%Y%m%d_%H%M%S_%f")
+        session_dir = os.path.join(self.DEBUG_DIR, ts)
+        os.makedirs(session_dir, exist_ok=True)
+        return session_dir
 
     def extract(
         self, image_bytes: bytes, content_type: Optional[str] = None
@@ -89,10 +113,14 @@ class VinExtractor(BaseExtractor):
             )
 
         try:
+            debug_session = self._create_debug_session()
+
             logger.debug(
                 "VIN extraction input: %d bytes, content_type=%s",
                 len(image_bytes), content_type,
             )
+            if debug_session:
+                self._save_debug_image(debug_session, "01_original.jpg", image_bytes)
 
             # Apply VIN-optimized preprocessing
             preprocessing_result = vin_preprocessor.preprocess(image_bytes)
@@ -100,6 +128,10 @@ class VinExtractor(BaseExtractor):
             logger.debug(
                 "Preprocessing steps: %s", preprocessing_result.preprocessing_applied
             )
+            if debug_session:
+                self._save_debug_image(
+                    debug_session, "02_preprocessed_adaptive.png", preprocessed_bytes
+                )
 
             # Perform OCR with VIN-optimized settings
             raw_text, word_confidences = self._perform_ocr(preprocessed_bytes)
@@ -121,6 +153,11 @@ class VinExtractor(BaseExtractor):
                     "Otsu preprocessing steps: %s",
                     otsu_result.preprocessing_applied,
                 )
+                if debug_session:
+                    self._save_debug_image(
+                        debug_session, "03_preprocessed_otsu.png",
+                        otsu_result.image_bytes,
+                    )
 
                 raw_text, word_confidences = self._perform_ocr(otsu_result.image_bytes)
                 logger.debug("Otsu PSM 6 raw text: '%s'", raw_text)