Fix Simple Mapping not applying during sync - preserve id field

Root cause: The UUID (id) field was being stripped when saving path mapping:
- Backend ReplacementRule model was missing id field
- Frontend pathMappingToApi() didn't include id in conversion
- Backend update_path_mapping endpoint didn't save id

Changes:
- Add id field to ReplacementRule model in main.py
- Include id when saving path mapping rules in update_path_mapping
- Include id in frontend pathMappingToApi conversion

Co-authored-by: Koha9 <36852125+Koha9@users.noreply.github.com>

frontend/services/api.ts

@@ -82,7 +82,7 @@ const mapPathMappingConfig = (data: any): PathMappingConfig => {
 // Helper function to convert PathMappingConfig to API format
 const pathMappingToApi = (config: PathMappingConfig) => {
   const rulesToApi = (rules: ReplacementRule[]) =>
-    rules.map(({ search, replace }) => ({ search, replace }));
+    rules.map(({ id, search, replace }) => ({ id, search, replace }));
 
   return {
     mode: config.mode,