feat: add VIN photo OCR pipeline (refs #67)

Implement VIN-specific OCR extraction with optimized preprocessing:

- Add POST /extract/vin endpoint for VIN extraction
- VIN preprocessor: CLAHE, deskew, denoise, adaptive threshold
- VIN validator: check digit validation, OCR error correction (I->1, O->0)
- VIN extractor: PSM modes 6/7/8, character whitelist, alternatives
- Response includes confidence, bounding box, and alternatives
- Unit tests for validator and preprocessor
- Integration tests for VIN extraction endpoint

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

ocr/app/validators/vin_validator.py

@@ -0,0 +1,259 @@
+"""VIN validation with check digit verification and OCR error correction."""
+import re
+from dataclasses import dataclass
+from typing import Optional
+
+
+@dataclass
+class VinValidationResult:
+    """Result of VIN validation."""
+
+    is_valid: bool
+    vin: str
+    confidence_adjustment: float
+    error: Optional[str] = None
+
+
+class VinValidator:
+    """Validates and corrects VIN strings."""
+
+    # VIN character set (excludes I, O, Q)
+    VALID_CHARS = set("ABCDEFGHJKLMNPRSTUVWXYZ0123456789")
+
+    # Common OCR misreads and their corrections
+    TRANSLITERATION = {
+        "I": "1",
+        "O": "0",
+        "Q": "0",
+        "i": "1",
+        "o": "0",
+        "q": "0",
+        "l": "1",
+        "L": "1",
+        "B": "8",  # Sometimes confused
+        "S": "5",  # Sometimes confused
+    }
+
+    # Weights for check digit calculation (positions 1-17)
+    CHECK_WEIGHTS = [8, 7, 6, 5, 4, 3, 2, 10, 0, 9, 8, 7, 6, 5, 4, 3, 2]
+
+    # Character to value mapping for check digit
+    CHAR_VALUES = {
+        "A": 1,
+        "B": 2,
+        "C": 3,
+        "D": 4,
+        "E": 5,
+        "F": 6,
+        "G": 7,
+        "H": 8,
+        "J": 1,
+        "K": 2,
+        "L": 3,
+        "M": 4,
+        "N": 5,
+        "P": 7,
+        "R": 9,
+        "S": 2,
+        "T": 3,
+        "U": 4,
+        "V": 5,
+        "W": 6,
+        "X": 7,
+        "Y": 8,
+        "Z": 9,
+        "0": 0,
+        "1": 1,
+        "2": 2,
+        "3": 3,
+        "4": 4,
+        "5": 5,
+        "6": 6,
+        "7": 7,
+        "8": 8,
+        "9": 9,
+    }
+
+    # Modern VIN pattern (1981+): exactly 17 alphanumeric, no I/O/Q
+    MODERN_VIN_PATTERN = re.compile(r"^[A-HJ-NPR-Z0-9]{17}$")
+
+    # Pre-1981 VIN pattern: 11-17 characters
+    LEGACY_VIN_PATTERN = re.compile(r"^[A-HJ-NPR-Z0-9]{11,17}$")
+
+    def correct_ocr_errors(self, vin: str) -> str:
+        """
+        Apply common OCR error corrections to a VIN string.
+
+        Args:
+            vin: Raw VIN string from OCR
+
+        Returns:
+            Corrected VIN string
+        """
+        corrected = vin.upper().strip()
+
+        # Remove any spaces or dashes (common in formatted VINs)
+        corrected = corrected.replace(" ", "").replace("-", "")
+
+        # Apply transliteration for common OCR errors
+        result = []
+        for char in corrected:
+            if char in self.TRANSLITERATION:
+                result.append(self.TRANSLITERATION[char])
+            else:
+                result.append(char)
+
+        return "".join(result)
+
+    def calculate_check_digit(self, vin: str) -> Optional[str]:
+        """
+        Calculate the check digit (position 9) for a VIN.
+
+        Args:
+            vin: 17-character VIN string
+
+        Returns:
+            Expected check digit character, or None if calculation fails
+        """
+        if len(vin) != 17:
+            return None
+
+        try:
+            total = 0
+            for i, char in enumerate(vin.upper()):
+                if i == 8:  # Skip check digit position
+                    continue
+                value = self.CHAR_VALUES.get(char)
+                if value is None:
+                    return None
+                total += value * self.CHECK_WEIGHTS[i]
+
+            remainder = total % 11
+            if remainder == 10:
+                return "X"
+            return str(remainder)
+        except (KeyError, ValueError):
+            return None
+
+    def validate_check_digit(self, vin: str) -> bool:
+        """
+        Validate the check digit of a VIN.
+
+        Args:
+            vin: 17-character VIN string
+
+        Returns:
+            True if check digit is valid
+        """
+        if len(vin) != 17:
+            return False
+
+        expected = self.calculate_check_digit(vin)
+        if expected is None:
+            return False
+
+        return vin[8].upper() == expected
+
+    def validate(
+        self, vin: str, correct_errors: bool = True, allow_legacy: bool = False
+    ) -> VinValidationResult:
+        """
+        Validate a VIN string and optionally correct OCR errors.
+
+        Args:
+            vin: VIN string to validate
+            correct_errors: Whether to apply OCR error corrections
+            allow_legacy: Whether to allow pre-1981 VINs (11-17 chars)
+
+        Returns:
+            VinValidationResult with validation status and corrected VIN
+        """
+        if not vin:
+            return VinValidationResult(
+                is_valid=False, vin="", confidence_adjustment=-1.0, error="Empty VIN"
+            )
+
+        # Apply error corrections if enabled
+        corrected_vin = self.correct_ocr_errors(vin) if correct_errors else vin.upper()
+
+        # Check length
+        if len(corrected_vin) != 17:
+            if allow_legacy and 11 <= len(corrected_vin) <= 17:
+                # Legacy VIN - reduced confidence
+                if self.LEGACY_VIN_PATTERN.match(corrected_vin):
+                    return VinValidationResult(
+                        is_valid=True,
+                        vin=corrected_vin,
+                        confidence_adjustment=-0.2,
+                    )
+            return VinValidationResult(
+                is_valid=False,
+                vin=corrected_vin,
+                confidence_adjustment=-0.5,
+                error=f"Invalid length: {len(corrected_vin)} (expected 17)",
+            )
+
+        # Check character set
+        if not self.MODERN_VIN_PATTERN.match(corrected_vin):
+            invalid_chars = [c for c in corrected_vin if c not in self.VALID_CHARS]
+            return VinValidationResult(
+                is_valid=False,
+                vin=corrected_vin,
+                confidence_adjustment=-0.3,
+                error=f"Invalid characters: {invalid_chars}",
+            )
+
+        # Validate check digit
+        if self.validate_check_digit(corrected_vin):
+            # Valid check digit - boost confidence
+            return VinValidationResult(
+                is_valid=True, vin=corrected_vin, confidence_adjustment=0.1
+            )
+        else:
+            # Invalid check digit - could be OCR error or old VIN
+            return VinValidationResult(
+                is_valid=True,  # Still return as valid but with reduced confidence
+                vin=corrected_vin,
+                confidence_adjustment=-0.15,
+                error="Check digit validation failed",
+            )
+
+    def extract_candidates(
+        self, text: str, max_candidates: int = 5
+    ) -> list[tuple[str, int, int]]:
+        """
+        Extract VIN candidates from raw OCR text.
+
+        Args:
+            text: Raw OCR text
+            max_candidates: Maximum number of candidates to return
+
+        Returns:
+            List of (vin, start_pos, end_pos) tuples
+        """
+        # Pattern to find potential VIN sequences
+        # Allow some flexibility for OCR errors (include I, O, Q for correction later)
+        potential_vin_pattern = re.compile(r"[A-Z0-9IOQ]{11,17}", re.IGNORECASE)
+
+        candidates = []
+        for match in potential_vin_pattern.finditer(text.upper()):
+            candidate = match.group()
+            corrected = self.correct_ocr_errors(candidate)
+
+            # Only include if it could be a valid VIN after correction
+            if len(corrected) == 17 and self.MODERN_VIN_PATTERN.match(corrected):
+                candidates.append((corrected, match.start(), match.end()))
+
+        # Sort by likelihood of being valid (check digit validation)
+        def score_candidate(c: tuple[str, int, int]) -> int:
+            vin = c[0]
+            if self.validate_check_digit(vin):
+                return 0  # Best score
+            return 1
+
+        candidates.sort(key=score_candidate)
+        return candidates[:max_candidates]
+
+
+# Singleton instance
+vin_validator = VinValidator()