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/router/library.go

@@ -36,8 +36,8 @@ func registerLibraryRoutes(cfg *Config) {
 	adminLibrary.GET("/:id/folders", cfg.LibraryHandler.GetLibraryFolders)
 	adminLibrary.DELETE("/:id/folders", cfg.LibraryHandler.DeleteLibraryFolder)
 	adminLibrary.GET("/:id/stats", cfg.LibraryHandler.GetLibraryStats)
-	adminLibrary.GET("/:id/issues/list", processingIssuesHandler.ListProcessingIssues)
-	adminLibrary.GET("/:id/issues/stats", processingIssuesHandler.GetProcessingIssueStats)
+	adminLibrary.GET("/:id/issues/list", cfg.ProcessingIssuesHandler.ListProcessingIssues)
+	adminLibrary.GET("/:id/issues/stats", cfg.ProcessingIssuesHandler.GetProcessingIssueStats)
 	adminLibrary.POST("/:id/scan", func(c *echo.Context) error {
 		libraryID := c.Param("id")
 		scanReq := map[string]interface{}{