feat: render health endpoint + test script + pipeline fixes
- GET /api/worker/health/render: checks render-worker (thumbnail_rendering
queue), Blender availability via active_queues inspect, queue depth,
last render recency — returns ok/degraded/down status
- scripts/test_render_pipeline.py: integration test for full pipeline
(--health, --sample, --full modes)
- PLAN.md: appended Render Pipeline Fixes section with all B-Fixes
- LEARNINGS.md: documented 5 new learnings (queue mismatch, circular
import, 307 redirect, worker capability detection)
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Hartmut
381f44bc8b