Add SSH SFTP backend support

internal/config/ssh.go

@@ -0,0 +1,64 @@
+package config
+
+import (
+	"fmt"
+	"strings"
+
+	"gopkg.in/yaml.v3"
+)
+
+type HostKeyPolicy string
+
+const (
+	HostKeyPolicyStrict    HostKeyPolicy = "strict"
+	HostKeyPolicyAcceptNew HostKeyPolicy = "accept-new"
+	HostKeyPolicyOff       HostKeyPolicy = "off"
+)
+
+func (p *HostKeyPolicy) UnmarshalYAML(value *yaml.Node) error {
+	switch value.Kind {
+	case yaml.ScalarNode:
+	default:
+		return fmt.Errorf("host_key_policy must be a boolean or string")
+	}
+
+	switch value.Tag {
+	case "!!bool":
+		var enabled bool
+		if err := value.Decode(&enabled); err != nil {
+			return err
+		}
+		if enabled {
+			*p = HostKeyPolicyStrict
+		} else {
+			*p = HostKeyPolicyOff
+		}
+		return nil
+	case "!!str":
+		var raw string
+		if err := value.Decode(&raw); err != nil {
+			return err
+		}
+		normalized, ok := NormalizeHostKeyPolicy(raw)
+		if !ok {
+			return fmt.Errorf("host_key_policy must be strict, true, accept-new, off, or false")
+		}
+		*p = normalized
+		return nil
+	default:
+		return fmt.Errorf("host_key_policy must be a boolean or string")
+	}
+}
+
+func NormalizeHostKeyPolicy(value string) (HostKeyPolicy, bool) {
+	switch strings.ToLower(value) {
+	case "", string(HostKeyPolicyAcceptNew):
+		return HostKeyPolicyAcceptNew, true
+	case string(HostKeyPolicyStrict), "true":
+		return HostKeyPolicyStrict, true
+	case string(HostKeyPolicyOff), "false":
+		return HostKeyPolicyOff, true
+	default:
+		return "", false
+	}
+}