Share the node entry point boilerplate

[klangenk]

Motivation

Every node's main.py repeats the same twenty lines — read UVICORN_RELOAD, build the settings, construct the node, call uvicorn.run. There are eight copies across the three node repositories, and they have drifted: dfine_node parses settings with configargparse so each one is both a flag and an environment variable, while yolov5_node and classification_node hand-roll os.getenv with asserts and hardcode the host and port.

This puts it in one place, so a new node's entry point is the two lines that are actually its own.

Stacked on #89; review that one first.

Implementation

• Add helpers/entrypoint.py with node_parser(description=, legacy_env_prefix=) and run_node(app, args)
• A setting is named after its flag: --conf-threshold reads CONF_THRESHOLD. One declaration yields a flag, an environment variable, and a --help entry.
• legacy_env_prefix accepts a prefix an earlier version of a node required, with a warning naming the replacement, so adopting this breaks no deployment
• Add configargparse to the runtime dependencies
• Nine tests in tests/unit

Why unprefixed names

dfine_node required a DFINE_TRAINER_ / DFINE_DETECTOR_ prefix; the other two read bare names. Bare wins here because yolov5_node is the widely deployed one and dfine_node is still in beta — so the repositories running in the field keep every variable they already set, and the one repository that has to move is the one where moving is cheap. dfine_node passes legacy_env_prefix so its beta deployments keep working meanwhile.

--host and --port are the exception

They read NODE_HOST and NODE_PORT, not the names their flags imply. The bare HOST already means the loop's address — every deployment sets it, face_detection's compose file sets HOST=preview.learning-loop.ai — and a node adopting it would hand that to uvicorn and fail to bind. There is a test asserting exactly this.

Note

logging.basicConfig(...) in the trainers is deliberately not reproduced. helpers/log_conf.py configures the root logger at import, so basicConfig has been returning without doing anything — verified, not assumed.

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[klangenk]

Superseded by #94, which combines all three phases into one PR per repository. The commits are unchanged and still readable in order.