FEAT: Add Stale-While-Revalidate Mode To CacheRead

[MrEthical07]

Background:
Current cache behavior is strict hit or miss. There is no stale serving strategy during backend slowness.

Problem:
Without stale-while-revalidate, p95 and p99 can degrade sharply during transient backend latency spikes.

Scope:

• Extend cache manager and CacheRead policy to support stale-while-revalidate behavior.
• Keep authenticated-cache safety and vary-by requirements unchanged.
• Update behavior docs in docs/cache-guide.md after implementation.

Tasks:

• Add stale window and metadata handling in cache manager.
• Add policy flow for stale serve with background refresh.
• Add tests for stale hit, stale expired, refresh success, refresh failure.
• Ensure observability labels distinguish stale serve vs normal hit.

Acceptance Criteria:

• Stale responses can be served within configured stale window.
• Background refresh does not break request path.
• Authenticated cache invariants and validation rules are preserved.