fix: address second code review findings

- 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

internal/musicbrainz/api_test.go

@@ -242,7 +242,7 @@ func TestGetArtistReleaseGroups_Pagination(t *testing.T) {
 			// First page: return 100 results (full page, matching limit) to trigger pagination
 			xml := `<?xml version="1.0" encoding="UTF-8"?>
 <metadata xmlns="http://musicbrainz.org/ns/mmd-2.0#">
-  <release-group-list count="150">`
+  <release-group-list count="101">`
 			for i := 0; i < 100; i++ {
 				xml += `
     <release-group id="rg-page1-` + string(rune('0'+i%10)) + `" type="Album">
@@ -263,7 +263,7 @@ func TestGetArtistReleaseGroups_Pagination(t *testing.T) {
 			// Second page: return only 1 result (< limit, signaling last page)
 			w.Write([]byte(`<?xml version="1.0" encoding="UTF-8"?>
 <metadata xmlns="http://musicbrainz.org/ns/mmd-2.0#">
-  <release-group-list count="150">
+  <release-group-list count="101">
     <release-group id="rg-page2" type="Album">
       <title>Page 2 Album</title>
       <first-release-date>2021-01-01</first-release-date>