refactor: replace standard log with zap logger and optimize code patterns

- Migrate all log.Printf/log.Println calls to structured zap logging
- Use cmp.Or for cleaner default value handling
- Replace manual loops with slices.ContainsFunc/IndexFunc
- Add batch query methods (IsLikedBatch, IsFavoritedBatch) to solve N+1 problem
- Update JSON tags from omitempty to omitzero for numeric fields
- Use errgroup-style wg.Go for worker goroutines
- Remove duplicate casbin/v2 dependency (keeping v3)
- Add plans/ to gitignore

internal/cache/cache.go

@@ -1,6 +1,7 @@
 package cache
 
 import (
+	"cmp"
 	"context"
 	"encoding/json"
 	"math/rand"
@@ -123,17 +124,11 @@ func GetSettings() Settings {
 }
 
 func normalizeKey(key string) string {
-	if settings.KeyPrefix == "" {
-		return key
-	}
-	return settings.KeyPrefix + ":" + key
+	return cmp.Or(settings.KeyPrefix, "") + ":" + key
 }
 
 func normalizePrefix(prefix string) string {
-	if settings.KeyPrefix == "" {
-		return prefix
-	}
-	return settings.KeyPrefix + ":" + prefix
+	return cmp.Or(settings.KeyPrefix, "") + ":" + prefix
 }
 
 func SetWithJitter(c Cache, key string, value interface{}, ttl time.Duration, jitterRatio float64) {