feat(infra): hardening firewall + sysctl + ssh

- deny-by-default iptables/ip6tables: public cuma 22/80/443/4013,
  sisanya localhost + tailscale CGNAT; log FW-DROP rate-limited
- persist via netfilter-persistent (rules.v4/v6) + /usr/local/bin/firewall.sh
- sysctl hardening: syncookies, rp_filter, redirect off, kptr/dmesg
  restrict, protected links, somaxconn, tcp retries tuner
- sshd: PasswordAuthentication no (fix 50-cloud-init.conf override),
  PermitRootLogin prohibit-password, MaxAuthTries 4, LoginGraceTime 30,
  X11Forwarding no

infra/firewall/99-hardening.conf

@@ -0,0 +1,40 @@
+# OrangeVPS hardening sysctl — /etc/sysctl.d/99-hardening.conf
+# Network hardening
+net.ipv4.tcp_syncookies = 1
+net.ipv4.conf.all.rp_filter = 1
+net.ipv4.conf.default.rp_filter = 1
+net.ipv4.conf.all.accept_redirects = 0
+net.ipv4.conf.default.accept_redirects = 0
+net.ipv4.conf.all.send_redirects = 0
+net.ipv4.conf.default.send_redirects = 0
+net.ipv4.conf.all.accept_source_route = 0
+net.ipv4.conf.default.accept_source_route = 0
+net.ipv6.conf.all.accept_redirects = 0
+net.ipv6.conf.default.accept_redirects = 0
+net.ipv6.conf.all.accept_source_route = 0
+net.ipv6.conf.default.accept_source_route = 0
+
+# TCP hardening
+net.ipv4.tcp_max_syn_backlog = 4096
+net.ipv4.tcp_synack_retries = 2
+net.ipv4.tcp_syn_retries = 3
+net.ipv4.tcp_fin_timeout = 15
+net.ipv4.tcp_keepalive_time = 120
+net.ipv4.tcp_keepalive_intvl = 30
+net.ipv4.tcp_keepalive_probes = 5
+net.ipv4.tcp_tw_reuse = 1
+net.ipv4.ip_local_port_range = 1024 65535
+
+# Kernel hardening
+kernel.randomize_va_space = 2
+kernel.core_uses_pid = 1
+kernel.dmesg_restrict = 1
+kernel.kptr_restrict = 1
+kernel.yama.ptrace_scope = 2
+fs.protected_hardlinks = 1
+fs.protected_symlinks = 1
+fs.suid_dumpable = 0
+
+# Resource limits (SYN flood protection)
+net.core.somaxconn = 1024
+net.core.netdev_max_backlog = 4096