🔧 Set sane Gitea retention and cleanup defaults 🔧

Shorter Actions retention and enable Gitea's built-in cleanup cron
tasks. Existing configs receive missing keys only; user-set values
are never overwritten.

- rootfs/tmp/etc/gitea/app.ini: artifacts 30d, logs 90d, runs 180d; enable gc_lfs, git_gc_repos, delete_old_actions, delete_old_system_notices; explicit cleanup_actions/cleanup_action_runs
- rootfs/usr/local/etc/docker/init.d/08-gitea.sh: add __gitea_ini_default and apply retention/cron defaults to existing configs when missing
- README.md: document retention defaults
- TODO.AI.md: record the change and its verification

Claude-Session: https://claude.ai/code/session_01Kae22tmhzhmfHyqc34PZbx

README.md

@@ -142,6 +142,11 @@ public hostname and keep `GITEA_PROTO=https`; Gitea keeps listening on plain htt
 `Secure`, and the proxy must forward `Host` and `X-Forwarded-Proto`. Use `GITEA_ROOT_URL` for a
 non-standard port or sub-path.
 
+**Retention** — Actions artifacts are kept 30 days, job logs 90 days and run history 180 days.
+Gitea's cron also garbage-collects orphaned LFS objects daily, runs `git gc` weekly, and prunes
+activity-feed entries older than 1 year and system notices older than 90 days. Edit
+`/config/gitea/app.ini` to change these; existing values are never overwritten on startup.
+
 **act\_runner**
 
 | Variable | Default | Description |