fix: make mongodb import lazy in casbinMongoRule (type-only import)

[JohannesWill]

Problem

Since 1.10.0, importing typeorm-adapter throws Cannot find module 'mongodb' (MODULE_NOT_FOUND) for any consumer that does not have mongodb installed — even when using a different driver (PostgreSQL, SQLite, …).

src/index.ts re-exports casbinMongoRule, and src/casbinMongoRule.ts imports mongodb at the top level. mongodb is only an optional peer dependency of typeorm and merely a devDependency here, so it is absent in most production installs.

Fix

ObjectId is only used as a type (public id!: ObjectId;). Switching to a type-only import removes the runtime require('mongodb'): with emitDecoratorMetadata, TypeScript no longer emits a reference to mongodb.ObjectId.

-import { ObjectId } from 'mongodb';
+import type { ObjectId } from 'mongodb';

Verification

Compiled output no longer contains any reference to mongodb:

__decorate([
    ObjectIdColumn(),
    __metadata("design:type", Function)
], CasbinMongoRule.prototype, "id", void 0);

grep -c mongodb lib/casbinMongoRule.js → 0. Build (tsc) and lint pass.

MongoDB users are unaffected — TypeORM loads the mongodb driver lazily via its own PlatformTools when a Mongo DataSource is actually initialized.

Closes #81