feat(admin): documents, indexes and the plan seed

Adds admin's own documents, its unique indexes and the plan seed from
shared/license.

Licences are append-only -- a renewal writes a new row and supersedes the
old one -- because the history is the support tool. Plans are seeded with
$setOnInsert only, so a redeploy never stamps over staff edits to limits,
features or Paddle IDs.

admin_instances.instance_id unique is a correctness property, not an
optimisation: without it two customers could both claim one self-hosted
UUID and both be issued licences for it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

admin/cmd/main.go

@@ -13,6 +13,7 @@ import (
 	"github.com/joho/godotenv"
 	"github.com/mrhid6/vantage/admin/internal/config"
 	"github.com/mrhid6/vantage/admin/internal/db"
+	"github.com/mrhid6/vantage/admin/internal/models"
 )
 
 func main() {
@@ -31,6 +32,17 @@ func main() {
 	cancel()
 	log.Printf("connected: admin=%s control=%s", cfg.AdminDBName, cfg.ControlDBName)
 
+	idxCtx, idxCancel := context.WithTimeout(context.Background(), 30*time.Second)
+	if err := db.EnsureIndexes(idxCtx); err != nil {
+		idxCancel()
+		log.Fatalf("indexes: %v", err)
+	}
+	if err := models.SeedPlans(idxCtx); err != nil {
+		idxCancel()
+		log.Fatalf("plan seed: %v", err)
+	}
+	idxCancel()
+
 	srv := &http.Server{
 		Addr:              cfg.Addr,
 		Handler:           http.NotFoundHandler(), // replaced in Task 8