feat(phase7.3): workflow editor pipeline step nodes

- GET /api/workflows/pipeline-steps: returns all StepName enum values
  with category (input|processing|rendering|output) + descriptions;
  registered before /{workflow_id} to avoid path collision
- frontend/src/api/workflows.ts: getPipelineSteps(), PipelineStep
  and PipelineStepsResponse interfaces
- WorkflowEditor: PipelineStepsPanel showing steps grouped by category
  with collapsible accordion sections
- ConfigSidepanel: "Pipeline Step" select dropdown binds any node to a
  StepName; selected step description shown below dropdown
- Active workflow indicator: green dot next to is_active=true entries
- Improved empty state with descriptive copy

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/domains/rendering/workflow_router.py

@@ -1,16 +1,19 @@
 """Workflow definition CRUD API.
 
 Endpoints:
-  GET    /api/workflows/          — list all workflow definitions (admin/PM)
-  GET    /api/workflows/{id}      — get single definition (admin/PM)
-  POST   /api/workflows/          — create definition (admin only)
-  PUT    /api/workflows/{id}      — update definition (admin only)
-  DELETE /api/workflows/{id}      — delete definition (admin only)
-  GET    /api/workflows/{id}/runs — list runs for a definition (admin/PM)
+  GET    /api/workflows/                — list all workflow definitions (admin/PM)
+  GET    /api/workflows/pipeline-steps  — list available pipeline step definitions
+  GET    /api/workflows/{id}            — get single definition (admin/PM)
+  POST   /api/workflows/                — create definition (admin only)
+  PUT    /api/workflows/{id}            — update definition (admin only)
+  DELETE /api/workflows/{id}            — delete definition (admin only)
+  GET    /api/workflows/{id}/runs       — list runs for a definition (admin/PM)
 """
 import uuid
+from typing import Literal
 
 from fastapi import APIRouter, Depends, HTTPException
+from pydantic import BaseModel
 from sqlalchemy import select
 from sqlalchemy.orm import selectinload
 from sqlalchemy.ext.asyncio import AsyncSession
@@ -25,10 +28,88 @@ from app.domains.rendering.schemas import (
     WorkflowDefinitionOut,
     WorkflowRunOut,
 )
+from app.core.process_steps import StepName
+
+
+# ── Pipeline-step metadata helpers ──────────────────────────────────────────
+
+StepCategory = Literal["input", "processing", "rendering", "output"]
+
+_STEP_CATEGORIES: dict[StepName, StepCategory] = {
+    StepName.RESOLVE_STEP_PATH: "input",
+    StepName.OCC_OBJECT_EXTRACT: "processing",
+    StepName.OCC_GLB_EXPORT: "processing",
+    StepName.GLB_BBOX: "processing",
+    StepName.MATERIAL_MAP_RESOLVE: "processing",
+    StepName.AUTO_POPULATE_MATERIALS: "processing",
+    StepName.BLENDER_RENDER: "rendering",
+    StepName.THREEJS_RENDER: "rendering",
+    StepName.THUMBNAIL_SAVE: "output",
+    StepName.ORDER_LINE_SETUP: "processing",
+    StepName.RESOLVE_TEMPLATE: "processing",
+    StepName.BLENDER_STILL: "rendering",
+    StepName.BLENDER_TURNTABLE: "rendering",
+    StepName.OUTPUT_SAVE: "output",
+    StepName.EXPORT_GLB_GEOMETRY: "output",
+    StepName.EXPORT_GLB_PRODUCTION: "output",
+    StepName.EXPORT_BLEND: "output",
+    StepName.STL_CACHE_GENERATE: "processing",
+    StepName.NOTIFY: "output",
+}
+
+_STEP_DESCRIPTIONS: dict[StepName, str] = {
+    StepName.RESOLVE_STEP_PATH: "Locate the STEP file on disk from the CadFile record",
+    StepName.OCC_OBJECT_EXTRACT: "Extract part objects and metadata from the STEP file using cadquery/OCC",
+    StepName.OCC_GLB_EXPORT: "Convert STEP geometry to glTF/GLB via cadquery",
+    StepName.GLB_BBOX: "Compute bounding-box from the exported GLB for camera framing",
+    StepName.MATERIAL_MAP_RESOLVE: "Resolve raw part-material names to SCHAEFFLER library materials via alias table",
+    StepName.AUTO_POPULATE_MATERIALS: "Auto-create Material records for any newly discovered part names",
+    StepName.BLENDER_RENDER: "Render a thumbnail PNG using Blender (Cycles or EEVEE)",
+    StepName.THREEJS_RENDER: "Render a thumbnail PNG using Three.js / Playwright headless browser",
+    StepName.THUMBNAIL_SAVE: "Persist the rendered thumbnail bytes to the CadFile record",
+    StepName.ORDER_LINE_SETUP: "Validate and prepare an order line for rendering (check STEP path, output type)",
+    StepName.RESOLVE_TEMPLATE: "Look up the matching RenderTemplate for the order line's category + output type",
+    StepName.BLENDER_STILL: "Render a production still image (PNG) via Blender HTTP micro-service",
+    StepName.BLENDER_TURNTABLE: "Render all turntable animation frames via Blender HTTP micro-service",
+    StepName.OUTPUT_SAVE: "Upload the rendered output file to storage and create a MediaAsset record",
+    StepName.EXPORT_GLB_GEOMETRY: "Export a geometry-only GLB for the 3-D viewer (no materials)",
+    StepName.EXPORT_GLB_PRODUCTION: "Export a production GLB with full materials from the .blend template",
+    StepName.EXPORT_BLEND: "Save the production .blend file as a downloadable MediaAsset",
+    StepName.STL_CACHE_GENERATE: "Convert STEP → STL (low + high quality) and cache next to the STEP file",
+    StepName.NOTIFY: "Emit a user notification via the audit-log notification channel",
+}
+
+
+class PipelineStepOut(BaseModel):
+    name: str
+    label: str
+    category: StepCategory
+    description: str
+
+
+class PipelineStepsResponse(BaseModel):
+    steps: list[PipelineStepOut]
 
 router = APIRouter(prefix="/api/workflows", tags=["workflows"])
 
 
+@router.get("/pipeline-steps", response_model=PipelineStepsResponse)
+async def get_pipeline_steps(
+    _user: User = Depends(require_admin_or_pm),
+):
+    """Return all available pipeline step definitions for the workflow editor."""
+    steps = [
+        PipelineStepOut(
+            name=step.value,
+            label=step.value.replace("_", " ").title(),
+            category=_STEP_CATEGORIES.get(step, "processing"),
+            description=_STEP_DESCRIPTIONS.get(step, ""),
+        )
+        for step in StepName
+    ]
+    return PipelineStepsResponse(steps=steps)
+
+
 @router.get("", response_model=list[WorkflowDefinitionOut])
 async def list_workflows(
     _user: User = Depends(require_admin_or_pm),