feat: TTL per key or pattern

Description:
Introduce Time-To-Live (TTL) support for cached entries. TTL can be set via response headers (e.g. Cache-Control, X-TTL) or via regex rules in config.yaml.

Acceptance Criteria:

- Expired entries must be removed automatically or skipped when accessed.
- TTLs can be overridden by config rules.
- TTLs apply to both memory and backend caches.
