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/deployer/deployer.go

@@ -707,9 +707,19 @@ func (d *Deployer) computeVolumeMounts(projectID, stageName, imageTag string) []
 		return nil
 	}
 
+	// Get base volume path from settings.
+	basePath := ""
+	if settings, err := d.store.GetSettings(); err == nil {
+		basePath = settings.BaseVolumePath
+	}
+
 	mounts := make([]mount.Mount, 0, len(vols))
 	for _, vol := range vols {
 		source := vol.Source
+		// Prepend base path if source is relative (doesn't start with /).
+		if basePath != "" && !filepath.IsAbs(source) {
+			source = filepath.Join(basePath, source)
+		}
 		if vol.Mode == "isolated" {
 			source = filepath.Join(source, fmt.Sprintf("%s-%s", stageName, imageTag))
 		}