Make cache entry TTL configurable in seconds

[fa0311]

What

Adds a cacheTtlSeconds config option to control how long a cached
IP→country entry stays valid before it is refreshed from the API.

Previously the TTL was hardcoded to one month (numberOfHoursInMonth).
Now it is configurable in seconds, defaulting to 30 days when unset so
existing behavior is preserved. As before, the TTL is only enforced when
forceMonthlyUpdate is enabled.

Usage

cacheTtlSeconds: 3600   # refresh entries older than 1 hour
forceMonthlyUpdate: true

Notes

• The cache schema (ipEntry: Country + Timestamp) is unchanged, so
  no migration is needed — existing cache files keep working.
• The TTL is evaluated at request time against the entry's creation
  Timestamp, so changing cacheTtlSeconds also affects already-cached
  entries.