feat: implement Mistral BYOK feature for OCR processing with user profile integration

backend/sample.config.yaml

@@ -32,10 +32,9 @@ ai:
   api_key: "${AI_API_KEY}"
 
 # Mistral OCR for document parsing
+# Note: API key is set per-user in Profile settings (BYOK)
 mistral:
-  api_key: "${MISTRAL_API_KEY}"
   ocr_model: "mistral-ocr-latest"
   max_pages_per_request: 8
   max_retries: 2        # Max retry attempts per chunk
   timeout_secs: 120     # Request timeout in seconds
-