make caching opt-in

README.md

@@ -68,11 +68,12 @@ Optional:
 
 - `GITHUB_TOKEN`
 - `GITEA_TOKEN`
-- `CACHE_TTL_SECONDS` (default: `3600`)
-- `CACHE_DIR` (default: `./cache` locally, `/app/cache` in container)
+- `CACHE_TTL_SECONDS` and `CACHE_DIR` enable caching when both are set
 - `DEFAULT_THEME` (`light` or `dark`, default: `light`)
 - `SERVICE_TITLE` (default: `git-activity-merge`)
 
+If you leave `CACHE_TTL_SECONDS` and `CACHE_DIR` undefined, the app skips caching entirely.
+
 ### 3. Run
 
 ```bash