Replace compute-only run() with an immediate sync+scan followed by a
ticker-driven periodic loop, add the sync.interval config field (default
6h) with defaults and validation, and add tests for the scheduling logic.
- Add CacheTTL default (24h) in config applyDefaults — without this,
omitting cache_ttl from config silently defeats the entire caching
mechanism (TTL=0 means cached data is never served)
- Fix pagination to use total Count instead of checking if last page
was short — avoids wasting a rate-limit token when total count is
an exact multiple of 100
- Preserve user-set IsIgnored flags across re-syncs — previously,
DELETE+INSERT in the sync transaction reset all ignore flags to
false, losing user preferences on every cache-expiry re-sync
- Check context cancellation on cache-hit code path — previously,
ctx.Err() was not checked between cache check and returning cached
data, violating the cancellation contract
