feat: Add the api_tokens collection and its indexes

The unique index on token_hash is what makes authentication an indexed
lookup rather than a scan, so this builder is fatal on failure like
EnsureAuthIndexes rather than warning like the secrets one.

Registered in ScopedCollections so instance purge reaches it.

server/internal/services/migrate_instance.go

@@ -43,6 +43,7 @@ var ScopedCollections = []string{
 	"server_packages",
 	"vuln_findings",
 	"vuln_alert_rules",
+	"api_tokens",
 	"server_workloads",
 }
 

server/internal/services/tokenindexes.go

@@ -0,0 +1,39 @@
+package services
+
+import (
+	"context"
+	"time"
+
+	"gitea.hostxtra.co.uk/mrhid6/vantage/server/internal/db"
+	"go.mongodb.org/mongo-driver/v2/bson"
+	"go.mongodb.org/mongo-driver/v2/mongo"
+	"go.mongodb.org/mongo-driver/v2/mongo/options"
+)
+
+// EnsureAPITokenIndexes declares the indexes the token path depends on.
+//
+// The unique index on token_hash is a security property, not an optimisation:
+// it is what makes authentication a single indexed lookup rather than a scan,
+// and what makes two tokens hashing to one value impossible to store.
+//
+// Fatal on failure, like EnsureAuthIndexes and unlike the secrets and workflow
+// builders: without the unique index the auth path would still answer, which is
+// exactly the wrong kind of degradation.
+func EnsureAPITokenIndexes() error {
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+
+	if _, err := db.Col("api_tokens").Indexes().CreateOne(ctx, mongo.IndexModel{
+		Keys:    bson.D{{Key: "token_hash", Value: 1}},
+		Options: options.Index().SetUnique(true),
+	}); err != nil {
+		return err
+	}
+
+	if _, err := db.Col("api_tokens").Indexes().CreateOne(ctx, mongo.IndexModel{
+		Keys: bson.D{{Key: "instance_id", Value: 1}, {Key: "user_id", Value: 1}},
+	}); err != nil {
+		return err
+	}
+	return nil
+}