refactor(phase3): remove dead services + STL remnant cleanup

Phase 3.2 — Delete orphaned service directories:
  - blender-renderer/ (HTTP microservice replaced by render-worker subprocess)
  - threejs-renderer/ (replaced by render-worker)
  - flamenco/ (removed in migration 032, directory still existed on disk)

Phase 3.2 — Remove STL workflow remnants:
  - analytics.py: remove avg_stl_s from RenderTimeBreakdown schema (always None)
  - kpi_service.py: remove avg_stl_s from return dicts + update docstring
  - frontend/src/api/analytics.ts: remove avg_stl_s from RenderTimeBreakdown interface
  - admin.py: remove dead blender-renderer HTTP configure call (service gone)

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

backend/app/api/routers/admin.py

@@ -366,17 +366,8 @@ async def update_settings(
         await _save_setting(db, k, v)
     await db.commit()
 
-    # Propagate concurrency limit to blender-renderer immediately (no restart needed)
-    if body.blender_max_concurrent_renders is not None:
-        try:
-            import httpx
-            async with httpx.AsyncClient(timeout=3.0) as client:
-                await client.post(
-                    "http://blender-renderer:8100/configure",
-                    params={"max_concurrent": body.blender_max_concurrent_renders},
-                )
-        except Exception:
-            pass  # best-effort; setting is persisted in DB regardless
+    # Note: blender-renderer HTTP service removed; concurrency is now controlled
+    # via render-worker Docker concurrency setting (thumbnail_rendering queue).
 
     return _settings_to_out(await _load_settings(db))