#311 test(e2e): give the DLQ repository's sample events a user

Backend E2E has failed on main since #309 made EventMetadata.user_id required: test_dlq_repository.py builds its sample events from dicts without a user, and mypy can't see into those. One failure, 473 passed. This adds user_id to the three dicts; the test passes against real Mongo, Redis and Kafka locally. No other test or app code builds metadata from a dict.

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Summary by cubic

Fixes the Backend E2E failure on main caused by #309 requiring EventMetadata.user_id. The DLQ repository test builds sample events from dicts that lack a user_id, and mypy can't check into those dicts. Adds user_id to all three sample events; the test passes locally against real Mongo, Redis, and Kafka, and no other test or app code builds metadata from a dict.