migrate to uv + pyproject.toml

- root pyproject.toml replaces requirements.txt and requirements-worker.txt
  (worker = root + ffmpeg-python which root already had); test deps moved
  to [dependency-groups] dev
- core/gpu/pyproject.toml replaces core/gpu/requirements.txt; uses
  [tool.uv.sources] to pin torch/torchvision and paddlepaddle-gpu to their
  CUDA index URLs, replacing the manual reinstall dance from old comments
- Dockerfiles use uv sync --frozen against uv.lock for reproducible builds;
  PATH includes /app/.venv/bin so k8s manifests' bare uvicorn/celery
  commands resolve without wrapping in uv run
- core/gpu/run.sh local mode now does uv sync + uv run python server.py;
  errors out cleanly if uv is missing

core/gpu/Dockerfile

@@ -8,8 +8,11 @@ RUN apt-get update && apt-get install -y \
 
 WORKDIR /app
 
-COPY requirements.txt .
-RUN uv pip install --system --no-cache -r requirements.txt
+ENV PATH="/app/.venv/bin:$PATH"
+
+# uv.lock is generated on first build (dev box can't reach paddlepaddle's index)
+COPY pyproject.toml ./
+RUN uv sync --no-install-project --no-dev
 
 COPY . .