Mitigate Forgejo archive endpoint DoS from crawler abuse

Crawlers hitting /archive/ endpoints with unique commit SHAs generated 54GB
of git bundles in 2 days, pegging Forgejo at 43% CPU. Fix at multiple layers:

- Redirect archive requests to tailnet at Fly proxy (302)
- Expand robots.txt: block /users/, /*/archive/, /*/releases/download/
- Cache release artifact downloads at nginx (immutable, 7d TTL)
- Enable [cron.archive_cleanup] with 2h TTL and run-at-start

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ansible/roles/forgejo/templates/app.ini.j2

@@ -61,6 +61,12 @@ MIN_INTERVAL = 10m
 [cron.update_checker]
 ENABLED = false
 
+[cron.archive_cleanup]
+ENABLED = true
+RUN_AT_START = true
+SCHEDULE = @midnight
+OLDER_THAN = 2h
+
 [session]
 PROVIDER = {{ forgejo_session_provider }}