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/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 {