feat: add stats helper service

Periodically snapshots Gitea instance metrics from a read-only MySQL
user and writes stats.json into Gitea custom/public/assets for the
home page to fetch same-origin. Config via env vars only.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,10 @@
+# build artifacts
+/stats/stats
+*.bak
+*.bak.*
+*.tmp
+stats.json
+
+# local env / secrets — never commit
+.env
+*.env