feat: add PHP-FPM, nginx, SSL, and port monitoring

All four are opt-in via config file:
- FPM_STATUS_URL — PHP-FPM pool stats (active/idle workers, slow requests, max children reached, accepted conn)
- NGINX_STATUS_URL — nginx stub_status (active connections, reading/writing/waiting, accepts/handled/requests)
- SSL_DOMAINS — TLS cert expiry check per domain (days left, valid, expires_at)
- CHECK_PORTS — TCP port availability per port with known service name mapping

Also refactors loadConfigFromFile to return *Config directly instead of a positional tuple.

payload.json

@@ -114,5 +114,47 @@
       "cpu_time": 890.12,
       "memory_bytes": 134217728
     }
-  ]
+  ],
+  "fpm": {
+    "pool": "www",
+    "active_processes": 3,
+    "idle_processes": 7,
+    "total_processes": 10,
+    "max_active_processes": 8,
+    "max_children_reached": 0,
+    "listen_queue": 0,
+    "max_listen_queue": 2,
+    "slow_requests": 0,
+    "accepted_conn": 84291
+  },
+  "ssl": [
+    {
+      "domain": "example.com",
+      "expires_at": "2026-06-01T12:00:00Z",
+      "days_left": 95,
+      "valid": true
+    },
+    {
+      "domain": "api.example.com",
+      "expires_at": "2026-06-01T12:00:00Z",
+      "days_left": 95,
+      "valid": true
+    }
+  ],
+  "ports": [
+    { "port": 22,   "open": true,  "service": "ssh" },
+    { "port": 80,   "open": true,  "service": "http" },
+    { "port": 443,  "open": true,  "service": "https" },
+    { "port": 3306, "open": true,  "service": "mysql" },
+    { "port": 6379, "open": false, "service": "redis" }
+  ],
+  "nginx": {
+    "active_connections": 42,
+    "reading": 1,
+    "writing": 8,
+    "waiting": 33,
+    "accepts": 1847293,
+    "handled": 1847293,
+    "requests": 3921847
+  }
 }