fix(docker): tear down matrix-started external DBs after smoke (#1516)

[natechadwick-intsof]

Summary

• After matrix-install-smoke finishes (pass or fail), stop external compose DBs that this process started (compose stop; no volume wipe).
• Reuse pre-existing operator-owned DB containers by default and leave them running unless --stop-db.
• Add --keep-db (leave DBs, destroy cells) and --stop-db (hard-stop all engaged external DBs); --keep still leaves cells and DBs for Playwright Layer 2.
• Document teardown policy in docker/README.md and harness help; unit tests cover ownership decision logic without live Docker.

Closes #1516

Test plan

• python -m pytest docker/scripts/test_matrix_install_smoke.py -v — 25 passed
• python docker/scripts/matrix-install-smoke.py --help shows --keep-db / --stop-db
• Optional live smoke: --product cms --db postgresql then confirm percussion-postgres is stopped when matrix started it
• Optional: pre-start postgres, run matrix without --stop-db, confirm container left running
• Optional: --keep leaves cell + DB; --keep-db destroys cell but leaves DB

Pre-PR verification

• Maven clean install: N/A — Python/docs only (no module pom.xml / Java sources changed).
• Erlang pre-commit: approve — docs/ai-generated/code-reviews/1516-matrix-install-smoke-db-teardown-erlang.md
• Unit tests: docker/scripts/test_matrix_install_smoke.py (25 passed)

Co-Authored by Grok Build (xAI) Grok 4.5 using grok-4.5 with agent Grok.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (4 files)

• docker/README.md - Documentation updates
• docker/scripts/matrix-install-smoke.py - DB ownership tracking and teardown logic
• docker/scripts/test_matrix_install_smoke.py - Policy and dry-run unit tests
• docs/ai-generated/code-reviews/1516-matrix-install-smoke-db-teardown-erlang.md - Pre-commit review doc

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_{Review guidance: REVIEW.md from base branch development}