refactor(task): extract billing and polling logic from controller to service layer

Restructure the task relay system for better separation of concerns:
- Extract task billing into service/task_billing.go with unified settlement flow
- Move task polling loop from controller to service/task_polling.go (supports Suno + video platforms)
- Split RelayTask into fetch/submit paths with dedicated retry logic (taskSubmitWithRetry)
- Add TaskDto, TaskResponse generics, and FetchReq to dto/task.go
- Add taskcommon/helpers.go for shared task adaptor utilities
- Remove controller/task_video.go (logic consolidated into service layer)
- Update all task adaptors (ali, doubao, gemini, hailuo, jimeng, kling, sora, suno, vertex, vidu)
- Simplify frontend task logs to use new TaskDto response format

relay/channel/task/taskcommon/helpers.go

@@ -0,0 +1,70 @@
+package taskcommon
+
+import (
+	"encoding/base64"
+	"fmt"
+
+	"github.com/QuantumNous/new-api/common"
+	"github.com/QuantumNous/new-api/setting/system_setting"
+)
+
+// UnmarshalMetadata converts a map[string]any metadata to a typed struct via JSON round-trip.
+// This replaces the repeated pattern: json.Marshal(metadata) → json.Unmarshal(bytes, &target).
+func UnmarshalMetadata(metadata map[string]any, target any) error {
+	if metadata == nil {
+		return nil
+	}
+	metaBytes, err := common.Marshal(metadata)
+	if err != nil {
+		return fmt.Errorf("marshal metadata failed: %w", err)
+	}
+	if err := common.Unmarshal(metaBytes, target); err != nil {
+		return fmt.Errorf("unmarshal metadata failed: %w", err)
+	}
+	return nil
+}
+
+// DefaultString returns val if non-empty, otherwise fallback.
+func DefaultString(val, fallback string) string {
+	if val == "" {
+		return fallback
+	}
+	return val
+}
+
+// DefaultInt returns val if non-zero, otherwise fallback.
+func DefaultInt(val, fallback int) int {
+	if val == 0 {
+		return fallback
+	}
+	return val
+}
+
+// EncodeLocalTaskID encodes an upstream operation name to a URL-safe base64 string.
+// Used by Gemini/Vertex to store upstream names as task IDs.
+func EncodeLocalTaskID(name string) string {
+	return base64.RawURLEncoding.EncodeToString([]byte(name))
+}
+
+// DecodeLocalTaskID decodes a base64-encoded upstream operation name.
+func DecodeLocalTaskID(id string) (string, error) {
+	b, err := base64.RawURLEncoding.DecodeString(id)
+	if err != nil {
+		return "", err
+	}
+	return string(b), nil
+}
+
+// BuildProxyURL constructs the video proxy URL using the public task ID.
+// e.g., "https://your-server.com/v1/videos/task_xxxx/content"
+func BuildProxyURL(taskID string) string {
+	return fmt.Sprintf("%s/v1/videos/%s/content", system_setting.ServerAddress, taskID)
+}
+
+// Status-to-progress mapping constants for polling updates.
+const (
+	ProgressSubmitted  = "10%"
+	ProgressQueued     = "20%"
+	ProgressInProgress = "30%"
+	ProgressComplete   = "100%"
+)