Add template build system with admin panel, async workers, and FlattenRootfs RPC

Introduces an end-to-end template building pipeline: admins submit a recipe
(list of shell commands) via the dashboard, a Redis-backed worker pool spins
up a sandbox, executes each command, and produces either a full snapshot
(with healthcheck) or an image-only template (rootfs flattened via a new
FlattenRootfs host-agent RPC). Build progress and per-step logs are persisted
to a new template_builds table and polled by the frontend.

Backend:
- New FlattenRootfs RPC (proto + host agent + sandbox manager)
- BuildService with Redis queue (BLPOP) and configurable worker pool (default 2)
- Admin-only REST endpoints: POST/GET /v1/admin/builds, GET /v1/admin/builds/{id}
- Migration for template_builds table with JSONB logs and recipe columns
- sqlc queries for build CRUD and progress updates

Frontend:
- /admin/templates page with Templates + Builds tabs
- Create Template dialog with recipe textarea, healthcheck, specs
- Build history with expandable per-step logs, status badges, progress bars
- Auto-polling every 3s for active builds
- AdminSidebar updated with Templates nav item

internal/db/models.go

@@ -147,6 +147,26 @@ type Template struct {
 	TeamID    string             `json:"team_id"`
 }
 
+type TemplateBuild struct {
+	ID           string             `json:"id"`
+	Name         string             `json:"name"`
+	BaseTemplate string             `json:"base_template"`
+	Recipe       []byte             `json:"recipe"`
+	Healthcheck  pgtype.Text        `json:"healthcheck"`
+	Vcpus        int32              `json:"vcpus"`
+	MemoryMb     int32              `json:"memory_mb"`
+	Status       string             `json:"status"`
+	CurrentStep  int32              `json:"current_step"`
+	TotalSteps   int32              `json:"total_steps"`
+	Logs         []byte             `json:"logs"`
+	Error        pgtype.Text        `json:"error"`
+	SandboxID    pgtype.Text        `json:"sandbox_id"`
+	HostID       pgtype.Text        `json:"host_id"`
+	CreatedAt    pgtype.Timestamptz `json:"created_at"`
+	StartedAt    pgtype.Timestamptz `json:"started_at"`
+	CompletedAt  pgtype.Timestamptz `json:"completed_at"`
+}
+
 type User struct {
 	ID           string             `json:"id"`
 	Email        string             `json:"email"`