feat: base volume path setting

Add global base_volume_path to settings. Relative volume source
paths are automatically prepended with the base path at deploy
time. Absolute paths are used as-is. Configurable in Settings >
General.

internal/store/models.go

@@ -53,6 +53,7 @@ type Settings struct {
 	NpmPassword      string `json:"npm_password"`
 	WebhookSecret    string `json:"webhook_secret"`
 	PollingInterval  string `json:"polling_interval"`
+	BaseVolumePath   string `json:"base_volume_path"`
 	UpdatedAt        string `json:"updated_at"`
 }