Add force update functionality for Ghost posts

- Add manual CI trigger 'force-update-ghost' for updating all posts
- Support FORCE_UPDATE environment variable in Rust code
- Implement post update logic via Ghost API PUT requests
- Add get_existing_post_id() function to find existing posts
- Update README with usage instructions
- Enhanced validation script to test new functionality

Usage:
- Normal: Only syncs new posts (default behavior)
- Force: FORCE_UPDATE=true updates ALL posts including existing ones

ghost-upload/.gitlab-ci.yml

@@ -13,3 +13,20 @@ publish:
     - pages
   rules:
     - if: "$CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH"
+
+# Manual trigger to force update all Ghost posts
+force-update-ghost:
+  stage: deploy
+  image: rust:latest
+  script:
+    - echo "🔄 Force updating all Ghost posts..."
+    - cd ./ghost-upload
+    - FORCE_UPDATE=true cargo run
+  needs:
+    - pages
+  rules:
+    - if: "$CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH"
+      when: manual
+      allow_failure: false
+  variables:
+    FORCE_UPDATE: "true"