fix(#59): local Docker integration test with snapshot comparison

[longieirl]

Summary

Aligns Docker integration testing with the existing Python integration test — same snapshot file, same logic, local-only.

Changes

• .gitignore: output_snapshot.json is now gitignored — personal to each developer's input PDFs, never committed. The copy on main is kept as a reference.
• test_output_snapshot.py: docstring updated to reflect local-only intent (remove 'committed to source control' language).
• docker_snapshot.py: new standalone script that builds the same snapshot structure as the Python test and either updates the baseline or diffs against it.
• Makefile: docker-integration target rewired to call docker_snapshot.py. First run: make docker-integration UPDATE=1 to create baseline. Subsequent runs: make docker-integration to validate.

Usage

# Create your local baseline (first time, or after an intentional change):
make docker-integration UPDATE=1

# Validate Docker output matches your baseline:
make docker-integration

# Python equivalent (same snapshot file):
pytest -m integration --snapshot-update  # baseline
pytest -m integration                    # validate

Type

• Bug fix

Testing

• Tests pass (coverage ≥ 91%)
• Manually tested
• make docker-integration passed locally

Checklist

• Code follows project style
• Self-reviewed
• No new warnings