fix(gitea): fall back to INITIAL_ADMIN_PASSWORD for gitea-init

Previously both compose files defaulted GITEA_ADMIN_PASSWORD to empty,
and scripts/gitea-init.sh silently skipped admin creation on blank input.
If config.sh failed to propagate the password (e.g. Docs Comments not
enabled, or --admin-password omitted), fresh installs ended up with a
Gitea container running but zero users — and the admin GUI's Gitea setup
wizard had no token to progress.

Changes:
- docker-compose.yml + docker-compose.prod.yml: GITEA_ADMIN_PASSWORD now
  falls back to INITIAL_ADMIN_PASSWORD when unset
- .env.example: declare GITEA_ADMIN_PASSWORD= with explanatory comment so
  users discover the override
- scripts/gitea-init.sh: silent skip becomes a loud WARN so a broken
  config is visible in compose logs

Bunker Admin

.env.example

@@ -231,6 +231,9 @@ GITEA_WEB_PORT=3030
 GITEA_SSH_PORT=2222
 # Admin user (auto-created on first boot by gitea-init.sh)
 GITEA_ADMIN_USER=admin
+# Leave blank to reuse INITIAL_ADMIN_PASSWORD (compose resolves the fallback).
+# Set only if you want a distinct password for the Gitea admin account.
+GITEA_ADMIN_PASSWORD=
 GITEA_DB_TYPE=mysql
 GITEA_DB_HOST=gitea-db:3306
 GITEA_DB_NAME=gitea