feat(db): replace postgres seeding service with mongodb

- Rewrites `DatabaseSeedingService` to work with a MongoDB `Db` instance.
- Removes table creation logic, as MongoDB collections are schemaless.
- Implements idempotent seeding using `bulkWrite` with `upsert` operations,
  preventing duplicate data on subsequent runs.
- Correctly handles the conversion of string IDs from fixtures to MongoDB
  `ObjectId` for the `_id` field.
- Ensures complex nested objects in fixtures are properly JSON-encoded
  before insertion.

Author: fulleni