Validate TTL and invalidation duration arithmetic consistently

[lan17]

Priority

P2 — Medium — important hardening or API work after the safety foundations.

Problem

Layer config accepts any positive safe-integer TTL in seconds. The local layer multiplies it by 1,000 without checking the result, while the Redis layer rejects an unsafe product. Invalidation also adds futureBufferMs to Redis time without an explicit safe-range contract.

Evidence:

• TTL validation accepts any positive safe integer
• Unchecked local multiplication
• Redis checks the multiplied TTL
• Redis-time plus future-buffer arithmetic

Acceptance criteria

• Define maximum accepted TTL and invalidation durations.
• Validate all products and sums before fallback results reach storage.
• Make local and Redis handling consistent for the same configuration.
• Add boundary tests around maximum accepted values and Number.MAX_SAFE_INTEGER.
• Preserve fail-open behavior for dynamic provider data while rejecting invalid static policy early.