Fix lint errors: add error checks for errcheck linter

- Add error check for JSON response in panic handler
- Remove duplicate defer CloseDB() call (handled in shutdown)
- Add error checks for all WriteField() calls in test files
- Add error checks for CreateFormFile() and Write() calls
- Fix golangci-lint Go 1.25 compatibility by installing from source

main.go

@@ -26,7 +26,7 @@ func main() {
 	if err != nil {
 		log.Fatalf("Failed to connect to database: %v", err)
 	}
-	defer services.CloseDB()
+	// Note: CloseDB is called explicitly in graceful shutdown, not in defer
 	log.Println("Database connected successfully")
 
 	// Initialize Firebase