feat(admin): module skeleton, config and two database connections

Adds the admin module: env config with fail-fast validation, two MongoDB
connections (its own vantage_admin database plus a narrow path into the
control plane), the boot sequence and the image.

Config refuses to start without a signing key, and both Mongo URIs must
name their database inline -- admin talks to two databases, so a bare
MONGO_DB would be ambiguous about which.

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

admin/internal/db/db.go

@@ -0,0 +1,61 @@
+// Package db holds admin's two MongoDB connections.
+//
+// Admin() is its own database and it owns every collection there. Control() is
+// the control plane's database, and admin's access to it is deliberately narrow:
+// it reads `instances` and `users`, and writes exactly three licence fields on
+// `instances`. Nothing here should ever grow a write path to another collection.
+package db
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"github.com/mrhid6/vantage/admin/internal/config"
+	"go.mongodb.org/mongo-driver/v2/mongo"
+	"go.mongodb.org/mongo-driver/v2/mongo/options"
+)
+
+var (
+	adminDB   *mongo.Database
+	controlDB *mongo.Database
+)
+
+func Connect(ctx context.Context, cfg config.Config) error {
+	ac, err := mongo.Connect(options.Client().ApplyURI(cfg.AdminMongoURI))
+	if err != nil {
+		return fmt.Errorf("connect admin mongo: %w", err)
+	}
+	if err := ac.Ping(ctx, nil); err != nil {
+		return fmt.Errorf("ping admin mongo: %w", err)
+	}
+	adminDB = ac.Database(cfg.AdminDBName)
+
+	cc, err := mongo.Connect(options.Client().ApplyURI(cfg.ControlMongoURI))
+	if err != nil {
+		return fmt.Errorf("connect control mongo: %w", err)
+	}
+	if err := cc.Ping(ctx, nil); err != nil {
+		return fmt.Errorf("ping control mongo: %w", err)
+	}
+	controlDB = cc.Database(cfg.ControlDBName)
+
+	// The control plane must already be deployed and migrated. Without the
+	// instances collection, injection would silently create it and write
+	// licence fields into a collection nothing reads.
+	names, err := controlDB.ListCollectionNames(ctx, map[string]any{"name": "instances"})
+	if err != nil {
+		return fmt.Errorf("inspect control database: %w", err)
+	}
+	if len(names) == 0 {
+		return fmt.Errorf("control database %q has no instances collection; deploy and migrate the control plane first", cfg.ControlDBName)
+	}
+	return nil
+}
+
+func Admin(name string) *mongo.Collection   { return adminDB.Collection(name) }
+func Control(name string) *mongo.Collection { return controlDB.Collection(name) }
+
+func Ctx() (context.Context, context.CancelFunc) {
+	return context.WithTimeout(context.Background(), 10*time.Second)
+}