feat: make cache defaults runtime-configurable

[snowyukitty]

Fixes #161

Summary

• Parse cache TTL settings strictly and fall back safely for malformed, non-positive, unsafe, or over-limit values.
• Support the documented CACHE_TTL_SECONDS and CACHE_NAMESPACE aliases while preserving REDIS_* precedence.
• Document the defaults and one-year TTL ceiling.
• Add focused coverage for aliases, precedence, invalid values, bounds, and namespace handling.

Testing

• pnpm exec vitest run (106 tests)
• pnpm exec tsc --noEmit
• pnpm exec eslint .
• pnpm exec next build --webpack

Additionally, at the time of submission of this PR:

• The referred issue is not blocked currently
• All unittests passed after changes were made

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Project	Deployment	Actions	Updated (UTC)
dev-impact	Ready	Preview	Aug 15, 2026 6:51pm

[O2sa]

✨ Thank you, @snowyukitty! Another great contribution merged! 🚀

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