uv run spawns uvicorn, which uses multiprocessing.spawn for hot reload.
The spawned subprocess starts a fresh Python interpreter that bypasses
uv's venv activation — so it never sees the venv's packages.
Fix: use a standalone python:3.12-slim dev stage with pip install -e .
directly into the system Python. No venv means the spawn subprocess uses
the same interpreter with the same packages. The editable install creates
a .pth file pointing to /app/src, so the mounted host source is live.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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