feat(queue): integrate Asynq + Redis Streams for background jobs and event-driven messaging

[GRACENOBLE]

Summary

• usecase.Enqueuer interface (Enqueue, Close) — the queue port, nil-safe when Redis is unavailable
• internal/infrastructure/queue/ — Asynq client, worker, TypeWelcomeEmail task constant + handler stub (real Mailjet delivery wired in feat: integrate Mailjet for transactional email (SMTP) #20); Asynqmon UI at /admin/queues in debug mode
• internal/infrastructure/streams/ — Redis Streams Producer (XADD) and consumer-group Consumer (XREADGROUP + XACK) for event fan-out (stream:user.created, stream:notification.sent)
• bootstrap.App gains Enqueuer field (nil when REDIS_URL unset — no new env vars)
• Worker goroutine wired in main.go following the ws.Hub pattern; cancelled before hub on graceful shutdown
• Queue unit tests run without Docker; integration test skips cleanly when Docker is unavailable
• Fixed stale docs: routing.md (NewServer signature, /metrics route), middleware.md (middleware chain order)
• New docs: backend/docs/queue.md, backend/docs/streams.md

Closes #18

Test plan

• go vet ./... — clean
• go test ./internal/infrastructure/queue/... — unit tests pass; integration test skips without Docker, passes with Docker
• go test ./internal/transport/... — all handler and middleware tests pass
• Start with REDIS_URL set: worker starts, GET /admin/queues returns Asynqmon UI
• Enqueue a TypeWelcomeEmail task and verify it appears in worker logs
• Publish a UserCreatedEvent to stream:user.created and verify XRange shows the entry
• Start without REDIS_URL: server starts normally, queue/streams features silently disabled

[coderabbitai]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 9a80a0ce-726d-4534-a01a-dedc8a6de7b3

📥 Commits

Reviewing files that changed from the base of the PR and between 99c064f and 94c8d6b.

⛔ Files ignored due to path filters (1)

• backend/go.sum is excluded by !**/*.sum

📒 Files selected for processing (27)

• backend/cmd/api/main.go
• backend/docs/_index.md
• backend/docs/middleware.md
• backend/docs/queue.md
• backend/docs/routing.md
• backend/docs/streams.md
• backend/docs/swagger/docs.go
• backend/docs/swagger/swagger.json
• backend/docs/swagger/swagger.yaml
• backend/go.mod
• backend/internal/bootstrap/bootstrap.go
• backend/internal/infrastructure/queue/client.go
• backend/internal/infrastructure/queue/client_test.go
• backend/internal/infrastructure/queue/handlers.go
• backend/internal/infrastructure/queue/handlers_test.go
• backend/internal/infrastructure/queue/tasks.go
• backend/internal/infrastructure/queue/worker.go
• backend/internal/infrastructure/streams/consumer.go
• backend/internal/infrastructure/streams/events.go
• backend/internal/infrastructure/streams/producer.go
• backend/internal/infrastructure/streams/producer_test.go
• backend/internal/server/server.go
• backend/internal/transport/handlers/handler.go
• backend/internal/transport/handlers/health_handler_test.go
• backend/internal/transport/handlers/queue_handler.go
• backend/internal/transport/handlers/routes.go
• backend/internal/usecase/enqueuer.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/18-asynq-redis-streams

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