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/api/routers/orders.py

@@ -954,12 +954,14 @@ async def cancel_line_render(
     cancelled_backend = line.render_backend_used or "celery"
     errors: list[str] = []
 
-    # Revoke Celery task (best-effort)
+    # Revoke Celery task (best-effort) using real task ID from job document
     try:
         from app.tasks.celery_app import celery_app
-        celery_app.control.revoke(
-            f"render-{line_id}", terminate=True, signal="SIGTERM"
-        )
+        real_task_id = None
+        if line.render_job_doc:
+            real_task_id = line.render_job_doc.get("celery_task_id")
+        task_id = real_task_id or f"render-{line_id}"
+        celery_app.control.revoke(task_id, terminate=True, signal="SIGTERM")
     except Exception as exc:
         errors.append(f"Celery revoke failed: {str(exc)[:200]}")
 
@@ -1025,11 +1027,13 @@ async def cancel_order_renders(
     errors: list[str] = []
 
     for line in lines:
-        # Revoke Celery task (best-effort)
+        # Revoke Celery task using real task ID from job document
         try:
-            celery_app.control.revoke(
-                f"render-{line.id}", terminate=True, signal="SIGTERM"
-            )
+            real_task_id = None
+            if line.render_job_doc:
+                real_task_id = line.render_job_doc.get("celery_task_id")
+            task_id = real_task_id or f"render-{line.id}"
+            celery_app.control.revoke(task_id, terminate=True, signal="SIGTERM")
         except Exception:
             pass