fix(feeds): enforce freshness window

[lazyGPT07]

Summary

• normalize positive finite freshnessDays values
• filter stale, undated, invalid-date, and future-dated feeds when a freshness window is requested
• preserve existing behavior when no freshness window is supplied
• add a network-free regression test

Validation

• feed-discovery focused suite: 8 tests passed
• feed-discovery TypeScript check passed
• git diff --check passed

Closes #51