fix: only update last_checked when cache actually changes

Avoid unnecessary file modifications by only updating last_checked
when there are new releases added or old releases deleted.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/zotero_scraper/services/cache_builder.py

@@ -232,6 +232,8 @@ class ReleaseCacheBuilder:
 
         if not tags_to_process:
             logger.debug(f"{repo}: No new releases to process")
+            # Only update last_checked if there were deleted releases
+            if deleted_tags:
                 self.cache.update_repo_checked_time(repo)
             return {"new_releases": 0, "deleted_releases": len(deleted_tags)}
 
@@ -263,6 +265,8 @@ class ReleaseCacheBuilder:
                 self.cache.add_release(repo, cached)
                 logger.debug(f"{repo}@{tag}: Parse failed, cached as failed")
 
+        # Only update last_checked if there were actual changes
+        if new_count > 0 or deleted_tags:
             self.cache.update_repo_checked_time(repo)
 
         return {"new_releases": new_count, "deleted_releases": len(deleted_tags)}