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

internal/db/models.go

@@ -91,6 +91,7 @@ type Sandbox struct {
 	Vcpus        int32              `json:"vcpus"`
 	MemoryMb     int32              `json:"memory_mb"`
 	TimeoutSec   int32              `json:"timeout_sec"`
+	DiskSizeMb   int32              `json:"disk_size_mb"`
 	GuestIp      string             `json:"guest_ip"`
 	HostIp       string             `json:"host_ip"`
 	CreatedAt    pgtype.Timestamptz `json:"created_at"`