feat(admin): linking, relink and the scoped customer API

Every customer handler that names an instance resolves it through
ownedInstance, which returns 404 rather than 403 for another account's
instance -- a 403 confirms the instance exists, which is an existence
oracle over customer data.

The unique index on admin_instances.instance_id, not the pre-check, is what
actually prevents two accounts owning one instance. Relink issues a
replacement covering the REMAINING term, so it cannot be used to extend a
subscription, and the old licence is not revoked because offline
verification has no revocation -- its instance binding is what stops it.

The route table lands with the staff handlers in the next commit so every
commit builds.

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

admin/internal/licensing/link.go

@@ -0,0 +1,121 @@
+package licensing
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"time"
+
+	"github.com/google/uuid"
+	"github.com/mrhid6/vantage/admin/internal/audit"
+	"github.com/mrhid6/vantage/admin/internal/db"
+	"github.com/mrhid6/vantage/admin/internal/models"
+	"github.com/mrhid6/vantage/shared/license"
+	"go.mongodb.org/mongo-driver/v2/bson"
+	"go.mongodb.org/mongo-driver/v2/mongo"
+)
+
+var (
+	ErrBadUUID       = errors.New("that does not look like an instance ID")
+	ErrAlreadyLinked = errors.New("that instance ID is already linked to an account")
+	ErrRelinkLimit   = errors.New("relink limit reached for this term; contact support")
+)
+
+// LinkInstance attaches a self-hosted instance UUID to an account.
+//
+// The duplicate error deliberately does not say WHICH account holds it. It is a
+// small enumeration surface, but there is no reason to leave it open.
+func LinkInstance(ctx context.Context, accountID, instanceID, name string) (*models.Instance, error) {
+	if _, err := uuid.Parse(instanceID); err != nil {
+		return nil, ErrBadUUID
+	}
+
+	// A self-hosted UUID must not collide with a cloud instance either.
+	if n, err := db.Control("instances").CountDocuments(ctx, bson.M{"instance_id": instanceID}); err == nil && n > 0 {
+		return nil, ErrAlreadyLinked
+	}
+
+	inst := models.Instance{
+		InstanceID: instanceID,
+		AccountID:  accountID,
+		Name:       name,
+		Deployment: license.DeploymentSelfHosted,
+		Status:     models.StatusActive,
+		CreatedAt:  time.Now().UTC(),
+	}
+	if _, err := db.Admin("admin_instances").InsertOne(ctx, inst); err != nil {
+		if mongo.IsDuplicateKeyError(err) {
+			// The unique index is what actually prevents two accounts owning
+			// one instance. The check above is a nicety; this is the guarantee.
+			return nil, ErrAlreadyLinked
+		}
+		return nil, err
+	}
+
+	audit.Write(ctx, models.AuditEntry{
+		Actor: accountID, Action: "instance.linked", AccountID: accountID, Target: instanceID})
+	return &inst, nil
+}
+
+// Relink moves a licence to a rebuilt server's new UUID.
+//
+// The replacement covers the REMAINING term, not a fresh one — relinking is not
+// a way to extend a subscription.
+//
+// The old licence is not revoked, because offline verification has no
+// revocation. It simply no longer matches any UUID the customer controls, and
+// its binding stops it working on another machine anyway.
+func Relink(ctx context.Context, accountID, oldID, newID string, staff bool) (*models.License, error) {
+	if _, err := uuid.Parse(newID); err != nil {
+		return nil, ErrBadUUID
+	}
+
+	var inst models.Instance
+	if err := db.Admin("admin_instances").FindOne(ctx,
+		bson.M{"instance_id": oldID, "account_id": accountID}).Decode(&inst); err != nil {
+		return nil, ErrUnknownInstance
+	}
+
+	// The cap is a signal, not a defence. Its job is to put a human in front of
+	// the fourth attempt, so staff bypass it.
+	if !staff && inst.RelinkCount >= models.MaxRelinksPerTerm {
+		return nil, ErrRelinkLimit
+	}
+
+	if n, err := db.Admin("admin_instances").CountDocuments(ctx, bson.M{"instance_id": newID}); err == nil && n > 0 {
+		return nil, ErrAlreadyLinked
+	}
+
+	// Preserve the remaining term from the current licence.
+	remaining := time.Now().UTC().Add(models.GracePeriod)
+	var current models.License
+	if err := db.Admin("licenses").FindOne(ctx,
+		bson.M{"license_id": inst.CurrentLicense}).Decode(&current); err == nil {
+		remaining = current.ExpiresAt
+	}
+
+	if _, err := db.Admin("admin_instances").UpdateOne(ctx,
+		bson.M{"instance_id": oldID},
+		bson.M{"$set": bson.M{"instance_id": newID}, "$inc": bson.M{"relink_count": 1}}); err != nil {
+		if mongo.IsDuplicateKeyError(err) {
+			return nil, ErrAlreadyLinked
+		}
+		return nil, fmt.Errorf("relink: %w", err)
+	}
+
+	actor := accountID
+	if staff {
+		actor = "staff"
+	}
+	audit.Write(ctx, models.AuditEntry{
+		Actor: actor, Action: "instance.relinked", AccountID: accountID,
+		Target: newID, Detail: "was " + oldID})
+
+	return Issue(ctx, IssueInput{
+		InstanceID: newID,
+		Tier:       inst.Tier,
+		ExpiresAt:  remaining,
+		Reason:     models.ReasonRelink,
+		IssuedBy:   actor,
+	})
+}