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

backend/src/models/user/user.rs

@@ -44,6 +44,9 @@ pub struct Model {
     /// URL to profile avatar icon
     pub avatar_url: Option<String>,
 
+    /// User's own Mistral API key (BYOK - Bring Your Own Key)
+    pub mistral_api_key: Option<String>,
+
     pub created_at: DateTime,
     pub updated_at: DateTime,
 }