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/api/router.go

@@ -192,6 +192,9 @@ func (s *Server) Router() chi.Router {
 			r.Get("/settings", s.getSettings)
 			r.Get("/settings/npm-certificates", s.listNpmCertificates)
 
+			// Volume scope metadata (read-only).
+			r.Get("/volumes/scopes", s.listVolumeScopes)
+
 			// Stale container endpoints (read).
 			r.Get("/containers/stale", s.listStaleContainers)