feat: push-based monitoring with remote agent

- Add database migration 006 for agent_key, agent_installed, agent_install_path, agent_last_seen_at columns
- Add Server model methods: findByAgentKey, generateAgentKey, regenerateAgentKey, updateAgentLastSeen, setAgentInstalled
- Auto-generate agent_key on server creation
- Create AgentService for SSH-based install/uninstall of remote agent
- Create bin/agent.sh - bash agent script for remote servers
- Create install/agent-install.sh and install/agent-uninstall.sh
- Create install/servermanager-agent.service systemd unit
- Add POST /api/metrics/push endpoint (auth via agent_key)
- Add web routes for agent install/uninstall/regenerate-key
- Add agent status section to server detail view with install/uninstall modals
- Make MonitoringService::saveMetrics() public for reuse

src/Models/Server.php

@@ -122,6 +122,10 @@ class Server
             $data['ssh_key'] = null;
         }
 
+        if (empty($data['agent_key'])) {
+            $data['agent_key'] = $this->generateAgentKey();
+        }
+
         $data['created_at'] = date('Y-m-d H:i:s');
         $data['updated_at'] = date('Y-m-d H:i:s');
 
@@ -405,4 +409,47 @@ class Server
             [$userId]
         );
     }
+
+    public function findByAgentKey(string $key): ?array
+    {
+        $server = $this->db->fetch(
+            'SELECT * FROM servers WHERE agent_key = ? AND status = \'active\'',
+            [$key]
+        );
+        if ($server) {
+            $server = $this->encryption->decryptArray($server, $this->encryptedFields);
+        }
+        return $server;
+    }
+
+    public function generateAgentKey(): string
+    {
+        return bin2hex(random_bytes(32));
+    }
+
+    public function regenerateAgentKey(int $id): string
+    {
+        $key = $this->generateAgentKey();
+        $this->db->update('servers', [
+            'agent_key' => $key,
+            'updated_at' => date('Y-m-d H:i:s'),
+        ], 'id = ?', [$id]);
+        return $key;
+    }
+
+    public function updateAgentLastSeen(int $id): bool
+    {
+        return $this->db->update('servers', [
+            'agent_last_seen_at' => date('Y-m-d H:i:s'),
+        ], 'id = ?', [$id]) > 0;
+    }
+
+    public function setAgentInstalled(int $id, bool $installed, ?string $path = null): bool
+    {
+        return $this->db->update('servers', [
+            'agent_installed' => $installed ? 1 : 0,
+            'agent_install_path' => $path,
+            'updated_at' => date('Y-m-d H:i:s'),
+        ], 'id = ?', [$id]) > 0;
+    }
 }