feat: volume scopes redesign — replace shared/isolated with 6 scopes

Replace confusing shared/isolated volume modes with explicit scopes:
- instance: per-deploy isolated directory
- stage: shared within a stage across deploys
- project: shared across all stages
- project_named: named group within a project
- named: global named volume across projects
- ephemeral: tmpfs in-memory mount

Includes schema migration (shared→project, isolated→instance),
backward-compatible deployer resolution, scope metadata API endpoint,
and redesigned volume editor UI with scope guide cards and hints.

internal/deployer/bluegreen.go

@@ -74,7 +74,7 @@ func (d *Deployer) blueGreenDeploy(
 	containerName := docker.ContainerName(project.Name, stage.Name, imageTag)
 	portStr := fmt.Sprintf("%d/tcp", project.Port)
 	envVars := d.mergeEnvVars(project, stage.ID)
-	mounts := d.computeVolumeMounts(project.ID, stage.Name, imageTag, settings.BaseVolumePath)
+	mounts := d.computeVolumeMounts(project.ID, project.Name, stage.Name, imageTag, settings.BaseVolumePath)
 
 	containerCfg := docker.ContainerConfig{
 		Name:         containerName,