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/internal/db/db.go

@@ -12,6 +12,7 @@ import (
 	"time"
 
 	"github.com/mrhid6/vantage/admin/internal/config"
+	"go.mongodb.org/mongo-driver/v2/bson"
 	"go.mongodb.org/mongo-driver/v2/mongo"
 	"go.mongodb.org/mongo-driver/v2/mongo/options"
 )
@@ -59,3 +60,54 @@ func Control(name string) *mongo.Collection { return controlDB.Collection(name)
 func Ctx() (context.Context, context.CancelFunc) {
 	return context.WithTimeout(context.Background(), 10*time.Second)
 }
+
+// EnsureIndexes creates admin's unique indexes.
+//
+// These are a correctness property, not an optimisation. In particular
+// admin_instances.instance_id unique is what stops the same self-hosted UUID
+// being linked to two accounts — without it, two customers could both claim one
+// instance and both be issued licences for it.
+func EnsureIndexes(ctx context.Context) error {
+	unique := []struct {
+		coll  string
+		field string
+	}{
+		{"accounts", "account_id"},
+		{"admin_instances", "instance_id"},
+		{"licenses", "license_id"},
+		{"plans", "tier"},
+		{"staff_users", "email"},
+		{"customer_users", "email"},
+	}
+	for _, u := range unique {
+		if _, err := Admin(u.coll).Indexes().CreateOne(ctx, mongo.IndexModel{
+			Keys:    bson.D{{Key: u.field, Value: 1}},
+			Options: options.Index().SetUnique(true).SetName(u.field + "_unique"),
+		}); err != nil {
+			return fmt.Errorf("index %s.%s: %w", u.coll, u.field, err)
+		}
+	}
+
+	// Sparse: a subscription exists before Paddle assigns an ID, so empty must
+	// not collide with empty.
+	if _, err := Admin("subscriptions").Indexes().CreateOne(ctx, mongo.IndexModel{
+		Keys:    bson.D{{Key: "paddle_subscription_id", Value: 1}},
+		Options: options.Index().SetUnique(true).SetSparse(true).SetName("paddle_subscription_id_unique"),
+	}); err != nil {
+		return fmt.Errorf("index subscriptions.paddle_subscription_id: %w", err)
+	}
+
+	for _, idx := range []struct {
+		coll string
+		keys bson.D
+	}{
+		{"licenses", bson.D{{Key: "instance_id", Value: 1}, {Key: "issued_at", Value: -1}}},
+		{"admin_instances", bson.D{{Key: "account_id", Value: 1}}},
+		{"admin_audit", bson.D{{Key: "created_at", Value: -1}}},
+	} {
+		if _, err := Admin(idx.coll).Indexes().CreateOne(ctx, mongo.IndexModel{Keys: idx.keys}); err != nil {
+			return fmt.Errorf("index %s: %w", idx.coll, err)
+		}
+	}
+	return nil
+}