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.

web/src/lib/i18n/en.json

@@ -120,15 +120,16 @@
   },
   "volumeEditor": {
     "title": "Volume Mounts",
-    "description": "Configure volume mounts for containers.",
-    "sharedDesc": "Shared mode uses the source path as-is for all instances.",
-    "isolatedDesc": "Isolated mode appends /{stage}-{tag}/ to the source, giving each instance its own directory.",
+    "description": "Configure volume mounts for containers. Choose a scope to control how volumes are shared between deploys.",
     "sourceHost": "Source (Host)",
     "targetContainer": "Target (Container)",
-    "mode": "Mode",
+    "scope": "Scope",
+    "nameColumn": "Name",
+    "namePlaceholder": "e.g. shared-db",
+    "requiresName": "requires name",
+    "noHostPath": "no host path",
+    "tmpfs": "tmpfs (in-memory)",
     "actions": "Actions",
-    "shared": "Shared",
-    "isolated": "Isolated",
     "edit": "Edit",
     "delete": "Delete",
     "save": "Save",