cli: add --api-port to hflow up

[chintondutta]

Fixes #7.

Current behavior

The runtime config already supports a custom API port -- RuntimeConfig.api_port: int = 8080, rendered into the bundle's .env as API_PORT -- but hflow up never passed it, so docs/RUNTIME.md's "Port 8080 is taken" entry had to document a manual .env-edit workaround.

Change

• Added --api-port (type int, default 8080, matching RuntimeConfig's own default) to the up subparser, forwarded into the RuntimeConfig(...) construction in _command_up.
• Updated the "Port 8080 is taken" troubleshooting entry in docs/RUNTIME.md to point at the flag -- while keeping the existing, still-true caveat: an existing bundle's .env is create-if-absent and never rewritten (so secrets and user edits survive re-renders), so --api-port only takes effect on a bundle's first render. For an already-rendered bundle, .env still needs a direct edit.

Testing

New test test_up_honors_api_port (tests/test_runtime_cli.py), following the existing test_up_renders_starts_and_prints pattern (compose subprocess and Airflow health stubbed, no Docker): runs hflow up --api-port 9090 and asserts the rendered bundle's .env contains API_PORT=9090 and the printed summary uses http://127.0.0.1:9090.

uv run pytest -q       # 298 passed, 3 skipped; unrelated: tests/test_ffmpeg.py fails/errors in this sandbox (no ffmpeg/ffprobe on PATH)
uv run ruff check --fix
uv run ruff format
uv run ty check
docker run --rm -v "$PWD:/data" -w /data lycheeverse/lychee:latest --no-progress --include-fragments \
  --exclude '^https://github\.com/Hebbian-Robotics/hflow/(issues|security/advisories/new)$' \
  --exclude-path references/mcap-spec.md \
  --exclude-path references/foxglove-CompressedVideo.proto .
# 230 Total, 0 Errors

(lychee wasn't on PATH in this sandbox; ran it via the official Docker image instead of installing.)

[github-actions]

👋 Hi @chintondutta — thanks for the contribution! To keep starter issues available
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You already have #52 open, so this one is being closed automatically.
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