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/store/models.go

@@ -109,13 +109,44 @@ type StageEnv struct {
 	UpdatedAt string `json:"updated_at"`
 }
 
+// VolumeScope defines the sharing scope for a volume mount.
+// Valid scopes: instance, stage, project, project_named, named, ephemeral.
+type VolumeScope string
+
+const (
+	VolumeScopeInstance     VolumeScope = "instance"
+	VolumeScopeStage        VolumeScope = "stage"
+	VolumeScopeProject      VolumeScope = "project"
+	VolumeScopeProjectNamed VolumeScope = "project_named"
+	VolumeScopeNamed        VolumeScope = "named"
+	VolumeScopeEphemeral    VolumeScope = "ephemeral"
+)
+
+// ValidVolumeScopes contains all valid scope values for validation.
+var ValidVolumeScopes = []VolumeScope{
+	VolumeScopeInstance, VolumeScopeStage, VolumeScopeProject,
+	VolumeScopeProjectNamed, VolumeScopeNamed, VolumeScopeEphemeral,
+}
+
+// IsValidVolumeScope returns true if the given string is a valid scope.
+func IsValidVolumeScope(s string) bool {
+	for _, v := range ValidVolumeScopes {
+		if string(v) == s {
+			return true
+		}
+	}
+	return false
+}
+
 // Volume represents a volume mount configuration for a project.
 type Volume struct {
 	ID        string `json:"id"`
 	ProjectID string `json:"project_id"`
 	Source    string `json:"source"`
 	Target   string `json:"target"`
-	Mode     string `json:"mode"` // shared or isolated
+	Mode     string `json:"mode,omitempty"` // legacy: shared/isolated — kept for DB compat
+	Scope    string `json:"scope"`          // instance, stage, project, project_named, named, ephemeral
+	Name     string `json:"name"`           // required for project_named and named scopes
 	CreatedAt string `json:"created_at"`
 	UpdatedAt string `json:"updated_at"`
 }