Add fail2ban dev test environment (Stage 0)

- Add bangui-sim filter (filter.d/bangui-sim.conf) matching the
  simulated authentication failure log format
- Add bangui-sim jail (jail.d/bangui-sim.conf) with maxretry=3,
  bantime=60s, findtime=120s, ignoreip safeguard, polling backend
- Mount Docker/logs/ into fail2ban container at /remotelogs/bangui
  in compose.debug.yml
- Add simulate_failed_logins.sh to write synthetic failure lines
- Add check_ban_status.sh with optional --unban flag
- Add dev-ban-test Makefile target for one-command smoke testing
- Write Docker/fail2ban-dev-config/README.md with setup and
  troubleshooting docs
- Update .gitignore to track custom config files while still
  excluding auto-generated linuxserver fail2ban files

.gitignore

@@ -93,7 +93,15 @@ Thumbs.db
 *~
 
 # ── Docker dev config ─────────────────────────
-Docker/fail2ban-dev-config/
+# Ignore auto-generated linuxserver/fail2ban config files,
+# but track our custom filter, jail, and documentation.
+Docker/fail2ban-dev-config/**
+!Docker/fail2ban-dev-config/README.md
+!Docker/fail2ban-dev-config/fail2ban/
+!Docker/fail2ban-dev-config/fail2ban/filter.d/
+!Docker/fail2ban-dev-config/fail2ban/filter.d/bangui-sim.conf
+!Docker/fail2ban-dev-config/fail2ban/jail.d/
+!Docker/fail2ban-dev-config/fail2ban/jail.d/bangui-sim.conf
 
 # ── Misc ──────────────────────────────────────
 *.log