feat: Nginx Proxy Manager admin panel section (#22-proxy)

- ProxyManager.php: install, start/stop/restart/reload, manage proxy hosts,
  write nginx configs, sync from accounts, setup script generator
- proxy.php API endpoint: full CRUD for proxy hosts + control/install/sync
- Admin panel: Nginx Proxy sidebar nav (Services section) with status cards,
  host table, add/edit/toggle/delete, auto-sync accounts, setup guide modal
- DB migration 003: proxy_hosts table + settings entries
- Sudoers: nginx systemctl/install rules for www-data
- Setup guide covers: local install, remote VM, automated script, vhost integration

db/migrations/003_proxy_hosts.sql

@@ -0,0 +1,19 @@
+-- Migration 003: Nginx Proxy Hosts table
+CREATE TABLE IF NOT EXISTS proxy_hosts (
+  id           INT AUTO_INCREMENT PRIMARY KEY,
+  account_id   INT UNSIGNED DEFAULT NULL,
+  domain       VARCHAR(255) NOT NULL,
+  upstream     VARCHAR(500) NOT NULL DEFAULT 'http://127.0.0.1:80',
+  ssl_enabled  TINYINT(1)   NOT NULL DEFAULT 0,
+  enabled      TINYINT(1)   NOT NULL DEFAULT 1,
+  custom_config TEXT         DEFAULT NULL,
+  notes        VARCHAR(500) DEFAULT NULL,
+  created_at   DATETIME     DEFAULT CURRENT_TIMESTAMP,
+  updated_at   DATETIME     DEFAULT NULL ON UPDATE CURRENT_TIMESTAMP,
+  INDEX (account_id),
+  UNIQUE KEY uq_domain (domain)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
+
+-- Panel settings for proxy mode
+INSERT INTO settings (`key`, `value`) VALUES ('proxy_mode', 'disabled') ON DUPLICATE KEY UPDATE `key`=`key`;
+INSERT INTO settings (`key`, `value`) VALUES ('proxy_auto_sync', '0') ON DUPLICATE KEY UPDATE `key`=`key`;