Pin dependency versions and add constraints.txt

Exact versions for direct dependencies, full pip freeze of the tested
image as constraints, both Dockerfiles install with -c constraints.txt.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Dockerfile.test

@@ -2,8 +2,8 @@ FROM python:3.11-slim
 
 WORKDIR /app
 
-COPY requirements.txt requirements-dev.txt ./
-RUN pip install --no-cache-dir -r requirements.txt -r requirements-dev.txt
+COPY requirements.txt requirements-dev.txt constraints.txt ./
+RUN pip install --no-cache-dir -c constraints.txt -r requirements.txt -r requirements-dev.txt
 
 COPY . .