Add CI integration test suite

[drdrew42]

Summary

• Modernize Docker build: multi-stage build (builder + runtime) cuts image size ~50%, adds .dockerignore, pins Perl deps in cpanfile, updates PG submodule to 2.20.1 (PG main with hotfixes)
• Add integration test suite: 36 bash+curl+jq tests across 4 categories — smoke, render parity (raw vs JWE/JWS), answer cycle (correct/wrong/partial scoring), and endpoint validation
• New GitHub Actions workflow (test.yml): builds container, runs PG unit tests (informational), then all integration suites with per-suite CI step visibility

Test categories

• Smoke (12 tests): health, editor UI, response structure, JWT presence, score defaults, instructor vs student response differences
• Render parity (8 tests): raw params and explicit JWE/JWS token paths produce identical rendered HTML (normalized to strip non-deterministic attributes)
• Answer cycle (8 tests): correct → score=1, wrong → score=0, partial → score=0.5, preview mode validation
• Endpoints (8 tests): static assets, catalog/tap file ops, error handling for missing problems, malformed JWTs, conflicting submit+preview

Running locally

bash t/ci/run-all.sh            # full build + test
bash t/ci/run-all.sh --no-build # skip Docker build (reuse existing image)

Test plan

• Fresh Docker build from ci-tests branch
• All 36 integration tests pass
• PG unit tests run (informational, don't gate CI)
• Verified render parity normalization distinguishes instructor vs student output

🤖 Generated with Claude Code