Add disk_size_mb, auto-expand base images, admin templates endpoint

Disk sizing:
- Add disk_size_mb column to sandboxes table (default 20480 = 20GB)
- Add disk_size_mb to CreateSandboxRequest proto, passed through the
  full chain: service → RPC → host agent → sandbox manager → devicemapper
- devicemapper.CreateSnapshot takes separate cowSizeBytes param so the
  sparse CoW file can be sized independently from the origin
- EnsureImageSizes() runs at host agent startup: expands any base image
  smaller than 20GB via truncate + resize2fs (sparse, no extra physical
  disk). Sandboxes then get the full 20GB via fast dm-snapshot path
- FlattenRootfs shrinks output images with resize2fs -M so stored
  templates are compact; EnsureImageSizes re-expands on next startup

Admin templates visibility:
- Add GET /v1/admin/templates endpoint listing all templates across teams
- Frontend admin templates page uses listAdminTemplates() instead of
  team-scoped listSnapshots()
- Platform templates (team_id = all-zeros UUID) now visible to all teams:
  GetTemplateByTeam, ListTemplatesByTeam, ListTemplatesByTeamAndType
  queries include platform team_id in WHERE clause

proto/hostagent/hostagent.proto

@@ -85,6 +85,10 @@ message CreateSandboxRequest {
   // TTL in seconds. Sandbox is auto-paused after this duration of
   // inactivity. 0 means no auto-pause.
   int32 timeout_sec = 4;
+
+  // Disk size in MB for the sparse CoW file. Limits how much data the
+  // sandbox can write beyond the base image. Default: 20480 (20 GB).
+  int32 disk_size_mb = 6;
 }
 
 message CreateSandboxResponse {