Add Redis caching layer`

[Nemenwq]

Dependencies: Issue #26.

Context:
Caching improves responsiveness and reduces load.

Implementation Notes:

• Focus on a few concrete caches first (e.g. rate oracle responses from Issue Implement conversion engine — price oracle integration` #37, merchant/profile lookups, API key validation).
• Ensure cache keys include merchant id where appropriate to avoid data leakage.

Scope:

• In scope: Shared Redis module + cache wrappers.
• Out of scope: Advanced cache invalidation strategies.

Files: backend/api_gateway/src/cache/