feat: 引入 Eino 框架，实现四阶段生成管线

基于 compose.Graph 编排 PromptBuilder → AssetGenerator → QualitySupervisor → FormatAdapter，
含质检重试分支（最多 3 次）和降级输出。推理层提供 mock 模式，可替换为真实 API。

backend/internal/service/pipeline.go

@@ -0,0 +1,102 @@
+package service
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/cloudwego/eino/compose"
+)
+
+const (
+	nodePromptBuilder     = "prompt_builder"
+	nodeAssetGenerator    = "asset_generator"
+	nodeQualitySupervisor = "quality_supervisor"
+	nodeFormatAdapter     = "format_adapter"
+)
+
+// NewGenerateGraph 创建四阶段生成管线 Graph。
+//
+//	START → PromptBuilder → AssetGenerator → QualitySupervisor
+//	                                            ├── pass → FormatAdapter → END
+//	                                            └── fail, retry<3 → PromptBuilder
+//	                                            └── fail, retry>=3 → FormatAdapter (降级)
+func NewGenerateGraph() (*compose.Graph[PipelineInput, PipelineOutput], error) {
+	g := compose.NewGraph[PipelineInput, PipelineOutput](
+		compose.WithGenLocalState(func(ctx context.Context) *PipelineState {
+			return &PipelineState{}
+		}),
+	)
+
+	// 添加节点
+	if err := g.AddLambdaNode(nodePromptBuilder, promptBuilderNode,
+		compose.WithStatePreHandler(promptBuilderPreHandler),
+		compose.WithStatePostHandler(promptBuilderPostHandler),
+	); err != nil {
+		return nil, fmt.Errorf("add %s node: %w", nodePromptBuilder, err)
+	}
+
+	if err := g.AddLambdaNode(nodeAssetGenerator, assetGeneratorNode,
+		compose.WithStatePostHandler(assetGeneratorPostHandler),
+	); err != nil {
+		return nil, fmt.Errorf("add %s node: %w", nodeAssetGenerator, err)
+	}
+
+	if err := g.AddLambdaNode(nodeQualitySupervisor, qualitySupervisorNode); err != nil {
+		return nil, fmt.Errorf("add %s node: %w", nodeQualitySupervisor, err)
+	}
+
+	if err := g.AddLambdaNode(nodeFormatAdapter, formatAdapterNode); err != nil {
+		return nil, fmt.Errorf("add %s node: %w", nodeFormatAdapter, err)
+	}
+
+	// 连线：正常路径
+	if err := g.AddEdge(compose.START, nodePromptBuilder); err != nil {
+		return nil, fmt.Errorf("add edge START->%s: %w", nodePromptBuilder, err)
+	}
+	if err := g.AddEdge(nodePromptBuilder, nodeAssetGenerator); err != nil {
+		return nil, fmt.Errorf("add edge %s->%s: %w", nodePromptBuilder, nodeAssetGenerator, err)
+	}
+	if err := g.AddEdge(nodeAssetGenerator, nodeQualitySupervisor); err != nil {
+		return nil, fmt.Errorf("add edge %s->%s: %w", nodeAssetGenerator, nodeQualitySupervisor, err)
+	}
+	if err := g.AddEdge(nodeFormatAdapter, compose.END); err != nil {
+		return nil, fmt.Errorf("add edge %s->END: %w", nodeFormatAdapter, err)
+	}
+
+	// 连线：质检分支（从 state.NextNode 读取路由目标）
+	if err := g.AddBranch(nodeQualitySupervisor, compose.NewGraphBranch(
+		func(ctx context.Context, _ PipelineInput) (string, error) {
+			var next string
+			_ = compose.ProcessState[*PipelineState](ctx, func(_ context.Context, state *PipelineState) error {
+				next = state.NextNode
+				return nil
+			})
+			return next, nil
+		},
+		map[string]bool{nodePromptBuilder: true, nodeFormatAdapter: true},
+	)); err != nil {
+		return nil, fmt.Errorf("add branch at %s: %w", nodeQualitySupervisor, err)
+	}
+
+	return g, nil
+}
+
+// RunPipeline 编译并执行生成管线
+func RunPipeline(ctx context.Context, in PipelineInput) (*PipelineOutput, error) {
+	g, err := NewGenerateGraph()
+	if err != nil {
+		return nil, fmt.Errorf("create graph: %w", err)
+	}
+
+	r, err := g.Compile(ctx, compose.WithMaxRunSteps(20))
+	if err != nil {
+		return nil, fmt.Errorf("compile graph: %w", err)
+	}
+
+	output, err := r.Invoke(ctx, in)
+	if err != nil {
+		return nil, fmt.Errorf("invoke pipeline: %w", err)
+	}
+
+	return &output, nil
+}