feat(backend): add Redis cache layer

[GRACENOBLE]

Summary

• Defines CacheService port interface in internal/usecase/ — the correct Clean Architecture home for interfaces the use case layer depends on
• Implements it in internal/infrastructure/cache/redis/ using go-redis/v9
• Bootstraps Redis via REDIS_URL env var — optional, probed with the same jittered-retry logic as Postgres, and closed on graceful shutdown

Architecture note

Stays faithful to Uncle Bob's Clean Architecture: domain (entities) and usecase (application logic + port interfaces) remain distinct layers. The hexagonal application/ports convention was considered and rejected in favour of this approach.

Test plan

• Set REDIS_URL in .env and run make watch — confirm bootstrap logs service ready for both postgres and redis
• Leave REDIS_URL empty — confirm server starts normally without Redis
• Run make test — existing tests still pass

[coderabbitai]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 6f1274dc-1031-44c5-b046-5ef8e1a4bb9f

📥 Commits

Reviewing files that changed from the base of the PR and between f97b3d1 and 1e00cc2.

⛔ Files ignored due to path filters (1)

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

📒 Files selected for processing (6)

• backend/.env.example
• backend/cmd/api/main.go
• backend/go.mod
• backend/internal/bootstrap/bootstrap.go
• backend/internal/infrastructure/cache/redis/cache.go
• backend/internal/usecase/cache.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch backend.cache

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