Add Mistral AI OCR script with test data and documentation

- ocr.php: two-step pipeline (mistral-ocr-latest + mistral-small-latest)
  extracts Serial Number, Model Number, and Date from part label photos
- input/: 5 test images of industrial part labels
- output/: corresponding YAML results
- README.md: full usage, setup, and troubleshooting docs
- .gitignore: excludes .env only
- .env.example: API key template

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

output/WIN_20260304_15_05_41_Pro.yaml

@@ -0,0 +1,19 @@
+---
+serial_number: Z1X6029782007
+model_number: B69199Q
+date: null
+source_file: WIN_20260304_15_05_41_Pro.jpg
+processed_at: "2026-03-04 17:26:32"
+raw_ocr: |
+  # POCLAIN
+  Hydraulics
+  
+  P/N: B69199Q
+  S/N: Z1X6029782007
+  
+  ![img-0.jpeg](img-0.jpeg)
+  
+  W/N: 0126
+  FN
+  
+  ![img-1.jpeg](img-1.jpeg)