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

database/migrations/006_agent_tokens.sql

@@ -0,0 +1,9 @@
+ALTER TABLE servers
+  ADD COLUMN agent_key VARCHAR(64) DEFAULT NULL AFTER ssh_key,
+  ADD COLUMN agent_installed TINYINT(1) NOT NULL DEFAULT 0 AFTER agent_key,
+  ADD COLUMN agent_install_path VARCHAR(255) DEFAULT NULL AFTER agent_installed,
+  ADD COLUMN agent_last_seen_at DATETIME DEFAULT NULL AFTER agent_install_path;
+
+CREATE UNIQUE INDEX idx_servers_agent_key ON servers(agent_key);
+
+UPDATE servers SET agent_key = LOWER(HEX(RANDOM_BYTES(32))) WHERE agent_key IS NULL;