feat(n8n): add n8n workflow automation container module

[demetere]

Summary

• Adds a new N8nContainer module for n8n workflow automation
• Uses the official docker.n8n.io/n8nio/n8n Docker image, exposing port 5678
• Health check via /healthz endpoint with @wait_container_is_ready decorator
• Zero external dependencies (stdlib urllib only)
• Provides get_url() and get_webhook_url() convenience methods

Closes #901

Implementation details

• N8nContainer extends DockerContainer, following the same pattern as VaultContainer / NginxContainer
• Disables telemetry (N8N_DIAGNOSTICS_ENABLED=false) by default for test environments
• Optional encryption_key parameter for credential encryption configuration
• Registered in pyproject.toml as n8n = [] (no extra pip dependencies)

Test plan

• test_docker_run_n8n — starts container, verifies /healthz returns 200 with {"status": "ok"}
• test_n8n_get_webhook_url — starts container, verifies webhook URL is correctly constructed
• All pre-commit hooks pass (ruff, ruff-format, trailing whitespace, end-of-files)

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