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/api/server.go

@@ -67,7 +67,7 @@ func New(
 	auditH := newAuditHandler(auditSvc)
 	statsH := newStatsHandler(statsSvc)
 	metricsH := newSandboxMetricsHandler(queries, pool)
-	buildH := newBuildHandler(buildSvc)
+	buildH := newBuildHandler(buildSvc, queries)
 
 	// OpenAPI spec and docs.
 	r.Get("/openapi.yaml", serveOpenAPI)
@@ -177,6 +177,7 @@ func New(
 		r.Use(requireJWT(jwtSecret))
 		r.Use(requireAdmin(queries))
 		r.Put("/teams/{id}/byoc", teamH.SetBYOC)
+		r.Get("/templates", buildH.ListTemplates)
 		r.Post("/builds", buildH.Create)
 		r.Get("/builds", buildH.List)
 		r.Get("/builds/{id}", buildH.Get)