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

db/queries/sandboxes.sql

@@ -1,6 +1,6 @@
 -- name: InsertSandbox :one
-INSERT INTO sandboxes (id, team_id, host_id, template, status, vcpus, memory_mb, timeout_sec)
-VALUES ($1, $2, $3, $4, $5, $6, $7, $8)
+INSERT INTO sandboxes (id, team_id, host_id, template, status, vcpus, memory_mb, timeout_sec, disk_size_mb)
+VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9)
 RETURNING *;
 
 -- name: GetSandbox :one