Add shutdown machine feature with configurable command

Backend:
- New migration: add shutdown_command TEXT column to machines table
- Machine model updated with ShutdownCommand field (Create/GetAll/GetByID/Update)
- MachineRequest/MachineResponse DTOs updated with shutdown_command field
- New ShutdownResponse DTO
- New POST /api/machines/{id}/shutdown handler via SSH
- Refactor sshmanager/testconn.go: extract dialSSH() helper shared with shutdown.go
- New sshmanager/shutdown.go: RunRemoteCommand with 15s timeout

Frontend:
- New shutdownMachine() API helper and ShutdownResponse type in client.ts
- New shutdown_command field in MachineForm
- Power button (amber) in machines table actions
- Shutdown confirmation modal with WoL warning notice
- shutdown_command input field in machine edit/create form
- Machine interface updated with shutdown_command and last_seen_at fields

internal/sshmanager/testconn.go

@@ -16,24 +16,24 @@ import (
 )
 
 type ConnResult struct {
-	Success    bool
-	Output     string
-	Error      string
+	Success     bool
+	Output      string
+	Error       string
 	Fingerprint string
 }
 
-func TestSSHConnection(ctx context.Context, host string, port int, user, privKeyPath, knownHostsPath string, strictHostKeyChecking bool) (*ConnResult, error) {
+func dialSSH(ctx context.Context, host string, port int, user, privKeyPath, knownHostsPath string, strictHostKeyChecking bool) (*ssh.Client, string, error) {
 	addr := fmt.Sprintf("%s:%d", host, port)
 
 	auths := []ssh.AuthMethod{}
 	if privKeyPath != "" {
 		key, err := os.ReadFile(privKeyPath)
 		if err != nil {
-			return nil, fmt.Errorf("reading private key: %w", err)
+			return nil, "", fmt.Errorf("reading private key: %w", err)
 		}
 		signer, err := ssh.ParsePrivateKey(key)
 		if err != nil {
-			return nil, fmt.Errorf("parsing private key: %w", err)
+			return nil, "", fmt.Errorf("parsing private key: %w", err)
 		}
 		auths = append(auths, ssh.PublicKeys(signer))
 	}
@@ -71,23 +71,27 @@ func TestSSHConnection(ctx context.Context, host string, port int, user, privKey
 	conn, err := ssh.Dial("tcp", addr, cfg)
 	if err != nil {
 		if strings.Contains(err.Error(), "known_hosts") || strings.Contains(err.Error(), "host key") {
-			return &ConnResult{
-				Success:     false,
-				Error:       fmt.Sprintf("host key verification failed: %v", err),
-				Fingerprint: capturedFingerprint,
-			}, nil
+			return nil, capturedFingerprint, fmt.Errorf("host key verification failed: %v", err)
 		}
+		return nil, capturedFingerprint, fmt.Errorf("connection failed: %v", err)
+	}
+	return conn, capturedFingerprint, nil
+}
+
+func TestSSHConnection(ctx context.Context, host string, port int, user, privKeyPath, knownHostsPath string, strictHostKeyChecking bool) (*ConnResult, error) {
+	conn, fingerprint, err := dialSSH(ctx, host, port, user, privKeyPath, knownHostsPath, strictHostKeyChecking)
+	if err != nil {
 		return &ConnResult{
 			Success:     false,
-			Error:       fmt.Sprintf("connection failed: %v", err),
-			Fingerprint: capturedFingerprint,
+			Error:       err.Error(),
+			Fingerprint: fingerprint,
 		}, nil
 	}
 	defer conn.Close()
 
 	session, err := conn.NewSession()
 	if err != nil {
-		return &ConnResult{Success: false, Error: fmt.Sprintf("session: %v", err), Fingerprint: capturedFingerprint}, nil
+		return &ConnResult{Success: false, Error: fmt.Sprintf("session: %v", err), Fingerprint: fingerprint}, nil
 	}
 	defer session.Close()
 
@@ -99,13 +103,13 @@ func TestSSHConnection(ctx context.Context, host string, port int, user, privKey
 		return &ConnResult{
 			Success:     false,
 			Error:       fmt.Sprintf("exec: %v, stderr: %s", err, stderr.String()),
-			Fingerprint: capturedFingerprint,
+			Fingerprint: fingerprint,
 		}, nil
 	}
 
 	return &ConnResult{
 		Success:     true,
 		Output:      stdout.String(),
-		Fingerprint: capturedFingerprint,
+		Fingerprint: fingerprint,
 	}, nil
 }