[kquinsland01]

The API image hardcodes its bind address and worker count in the Gunicorn command, so deployments must replace the entire command to enable IPv6 or tune process count.

This PR adds a few knobs to change this using env-vars:

• Small bit of python code into the existing Gunicorn configuration; preserving the existing defaults: 0.0.0.0:8080 and four workers.
• KEEP_API_BIND allows opt-in IPv6 (for example [::]:8080); the default bind honors PORT but an explicit bind includes its own port.
• KEEP_API_WORKERS adjusts API worker count; GUNICORN_CMD_ARGS and explicit CLI flags retain their usual higher precedence.

The existing entrypoint, Uvicorn worker class, preload, migration/provisioning hooks, and ARQ command remain intact.
Hosts without IPv6 support continue to work with the default configuration and use of IPv6 wildcard listeners will accept IPv4 connections only when the host supports dual-stack sockets.

Validation: nine tests pass against Gunicorn 23.0.0, covering image defaults, environment/CLI precedence, invalid worker counts, hook retention, ARQ/API worker separation, a real IPv4 default listener, and opt-in IPv6 plus host-supported dual-stack connections.

Resolves: #6968

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