refactor(services): Modernize Go code style in collection and filters services

Apply Go 1.18+ language features and modern style:

internal/services/collection_service.go:
- Use map[string]any instead of map[string]interface{} (Go 1.18+)
- Use range clause with single variable for iteration-only loops
- Replace if-else chains with switch statements for better readability
- Remove explicit type initialization for zero values

internal/services/filters.go:
- Add Err prefix to custom error variable for error naming convention

internal/router/library.go:
- Use cfg.ProcessingIssuesHandler instead of local processingIssuesHandler variable
- Ensures proper dependency injection through router config

These changes follow current Go best practices and improve code readability.

internal/services/collection_service.go

@@ -40,7 +40,7 @@ func NewCollectionService(db *database.Queries) *CollectionService {
 }
 
 // CreateCollection creates a new collection
-func (s *CollectionService) CreateCollection(ctx context.Context, userID uuid.UUID, name, description, color, icon string, autoAssignRules []Rule, viewSettings map[string]interface{}) (database.Collections, error) {
+func (s *CollectionService) CreateCollection(ctx context.Context, userID uuid.UUID, name, description, color, icon string, autoAssignRules []Rule, viewSettings map[string]any) (database.Collections, error) {
 	// Convert rules to JSONB ([]byte)
 	var rulesJSON []byte
 	if len(autoAssignRules) > 0 {
@@ -89,13 +89,13 @@ func (s *CollectionService) GetCollection(ctx context.Context, collectionID uuid
 }
 
 // GetCollectionWithBookCount gets a collection with book count
-func (s *CollectionService) GetCollectionWithBookCount(ctx context.Context, collectionID uuid.UUID) (map[string]interface{}, error) {
+func (s *CollectionService) GetCollectionWithBookCount(ctx context.Context, collectionID uuid.UUID) (map[string]any, error) {
 	collection, err := s.db.GetCollectionWithBookCount(ctx, pgtype.UUID{Bytes: collectionID, Valid: true})
 	if err != nil {
 		return nil, fmt.Errorf("failed to get collection: %v", err)
 	}
 
-	return map[string]interface{}{
+	return map[string]any{
 		"id":                uuid.UUID(collection.ID.Bytes).String(),
 		"user_id":           uuid.UUID(collection.UserID.Bytes).String(),
 		"name":              collection.Name,
@@ -120,7 +120,7 @@ func (s *CollectionService) GetUserCollections(ctx context.Context, userID uuid.
 }
 
 // UpdateCollection updates a collection
-func (s *CollectionService) UpdateCollection(ctx context.Context, collectionID uuid.UUID, name, description, color, icon string, autoAssignRules []Rule, viewSettings map[string]interface{}) (database.Collections, error) {
+func (s *CollectionService) UpdateCollection(ctx context.Context, collectionID uuid.UUID, name, description, color, icon string, autoAssignRules []Rule, viewSettings map[string]any) (database.Collections, error) {
 	// Convert rules to JSONB ([]byte)
 	var rulesJSON []byte
 	if len(autoAssignRules) > 0 {
@@ -224,7 +224,7 @@ func (s *CollectionService) EvaluateRules(mediaItem database.ListMediaItemsRow,
 	// Sort rules by priority (higher priority first)
 	sortedRules := make([]Rule, len(rules))
 	copy(sortedRules, rules)
-	for i := 0; i < len(sortedRules); i++ {
+	for i := range len(sortedRules) {
 		for j := i + 1; j < len(sortedRules); j++ {
 			if sortedRules[i].Priority < sortedRules[j].Priority {
 				sortedRules[i], sortedRules[j] = sortedRules[j], sortedRules[i]
@@ -274,9 +274,10 @@ func (s *CollectionService) EvaluateRules(mediaItem database.ListMediaItemsRow,
 		// Calculate confidence based on rule type and priority
 		if eval.Matches {
 			baseConfidence := 0.7
-			if rule.Field == "genre" || rule.Field == "series" {
+			switch rule.Field {
+			case "genre", "series":
 				baseConfidence = 0.9
-			} else if rule.Field == "author" || rule.Field == "publisher" {
+			case "author", "publisher":
 				baseConfidence = 0.8
 			}
 			// Add priority boost (0.01 per priority point, max 0.1)
@@ -487,7 +488,7 @@ func parseFloat(s string) (float64, error) {
 	var result float64
 	var sign float64 = 1
 	var divisor float64 = 1
-	var decimalPlaces int = 0
+	var decimalPlaces = 0
 	var seenDecimal bool
 
 	// Skip leading whitespace

internal/services/filters.go

@@ -11,7 +11,7 @@ import (
 	"github.com/jackc/pgx/v5/pgtype"
 )
 
-// Custom errors for saved filters
+// ErrFilterNotFound - Custom errors for saved filters
 var ErrFilterNotFound = errors.New("filter not found or access denied")
 
 type FiltersService struct {