Add Redis-backed integration tests for deduplication and shared state

[darthfork]

Parent issue: #52

Summary

Add integration coverage that uses a real Redis instance to validate delivery deduplication and shared state behavior.

Scope

• Run tests against a real Redis service in CI/local test harness.
• Validate X-GitHub-Delivery dedupe behavior.
• Validate workflow/job state transitions using the Redis-backed store.
• Validate shared-state behavior across multiple app instances where practical.

Scenarios

• Replayed delivery IDs do not double-count metrics.
• Workflow/job state persists correctly through Redis-backed lookups/updates.
• Two server instances sharing Redis do not both process the same delivery.
• Redis connection/config failures fail clearly.

Design notes

• Use a GitHub Actions Redis service or equivalent disposable Redis instance.
• Keep this suite separate from the no-dependency integration suite.
• Prefer black-box behavior assertions over store-implementation details.

Acceptance criteria

• Real Redis is used in test execution.
• Deduplication and shared state behavior are validated end to end.
• The suite is stable enough to run in CI.