feat(refactor/phase2): pipeline modularity + dead code removal

Phase 2.3 — Fix render cancellation (real Celery task ID):
  - orders.py cancel endpoints: read celery_task_id from render_job_doc
    instead of synthetic "render-{line_id}" which was a no-op
  - render_order_line_still_task: creates RenderJobDocument at task start,
    stores self.request.id as celery_task_id, writes step-level state
    (RESOLVE_STEP_PATH → BLENDER_STILL) back to order_lines.render_job_doc

Phase 3.1 — Remove Pillow overlay dead code:
  - blender_render.py: deleted 55-line Pillow post-processing block
    (lines 798-851, green bar + model name label)
  - transparent_bg=True is always passed; the else branch was unreachable
  - Removed mention from script docstring

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

backend/app/domains/rendering/tasks.py

@@ -395,14 +395,47 @@ def render_order_line_still_task(self, order_line_id: str, **params) -> dict:
     Wraps render_still_task logic but accepts order_line_id instead of step_path.
     On success, creates a MediaAsset record via publish_asset.
     """
+    import asyncio
+    from app.domains.rendering.job_document import RenderJobDocument, JobState
+    from app.core.process_steps import StepName
+
     log_task_event(self.request.id, f"Starting render_order_line_still_task: order_line={order_line_id}", "info")
+
+    # Initialise job document and store real Celery task ID
+    job_doc = RenderJobDocument.new(order_line_id=order_line_id, celery_task_id=self.request.id)
+    job_doc.set_state(JobState.RUNNING)
+
+    def _save_job_doc():
+        async def _run():
+            from app.database import AsyncSessionLocal
+            from app.domains.orders.models import OrderLine
+            from sqlalchemy import update as _upd
+            async with AsyncSessionLocal() as db:
+                await db.execute(
+                    _upd(OrderLine)
+                    .where(OrderLine.id == order_line_id)
+                    .values(render_job_doc=job_doc.to_dict())
+                )
+                await db.commit()
+        try:
+            asyncio.get_event_loop().run_until_complete(_run())
+        except Exception as _exc:
+            logger.debug("_save_job_doc failed: %s", _exc)
+
+    _save_job_doc()
+
+    job_doc.begin_step(StepName.RESOLVE_STEP_PATH)
     step_path_str, cad_file_id = _resolve_step_path_for_order_line(order_line_id)
     if not step_path_str:
+        job_doc.fail_step(StepName.RESOLVE_STEP_PATH, "product missing or has no linked CAD file")
+        job_doc.set_state(JobState.FAILED, error="Cannot resolve STEP path")
+        _save_job_doc()
         log_task_event(self.request.id, f"Failed: cannot resolve STEP path for order_line {order_line_id}", "error")
         raise RuntimeError(
             f"Cannot resolve STEP path for order_line {order_line_id}: "
             "product missing or has no linked CAD file"
         )
+    job_doc.finish_step(StepName.RESOLVE_STEP_PATH, output={"step_path": step_path_str})
 
     step = Path(step_path_str)
     output_dir = step.parent / "renders"
@@ -410,12 +443,24 @@ def render_order_line_still_task(self, order_line_id: str, **params) -> dict:
     output_path = output_dir / f"line_{order_line_id}.png"
 
     try:
+        job_doc.begin_step(StepName.BLENDER_STILL)
         from app.services.render_blender import render_still
         result = render_still(
             step_path=step,
             output_path=output_path,
             **params,
         )
+        job_doc.finish_step(
+            StepName.BLENDER_STILL,
+            output={"output_path": str(output_path), "duration_s": result.get("total_duration_s")},
+        )
+        job_doc.set_state(JobState.COMPLETED, result={
+            "output_path": str(output_path),
+            "duration_s": result.get("total_duration_s"),
+            "engine_used": result.get("engine_used"),
+        })
+        _save_job_doc()
+
         publish_asset.delay(
             order_line_id,
             "still",
@@ -438,6 +483,9 @@ def render_order_line_still_task(self, order_line_id: str, **params) -> dict:
         _update_workflow_run_status(order_line_id, "completed")
         return result
     except Exception as exc:
+        job_doc.fail_step(StepName.BLENDER_STILL, str(exc))
+        job_doc.set_state(JobState.FAILED, error=str(exc))
+        _save_job_doc()
         log_task_event(self.request.id, f"Failed: {exc}", "error")
         logger.error("render_order_line_still_task failed for %s: %s", order_line_id, exc)
         try: