CI: pin dependency resolution with constraints.txt

Fresh-venv installs started failing with pip ResolutionTooDeep because
requirements.txt is unpinned. Constrain CI and image builds to the
frozen, test-passing local set; requirements.txt stays the source of
intent.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/Dockerfile

@@ -1,7 +1,7 @@
 FROM python:3.11-slim
 WORKDIR /app
 RUN apt-get update && apt-get install -y --no-install-recommends poppler-utils && rm -rf /var/lib/apt/lists/*
-COPY requirements.txt .
-RUN pip install --no-cache-dir --pre -r requirements.txt
+COPY requirements.txt constraints.txt ./
+RUN pip install --no-cache-dir --pre -r requirements.txt -c constraints.txt
 COPY . .
 CMD ["uvicorn", "app.main:app", "--host", "0.0.0.0", "--port", "8000", "--reload"]