controller_settings: honor VE_DIRECT_PORT env var, prefer env over saved JSON

[Carl Chalmers (Clamps251)]

controller_settings.py's DEFAULTS dict hardcodes PORT: /dev/ttyUSB0. On any SPARROW Pi 5 build without a Victron VE.Direct cable — i.e. essentially every fresh field Pi we've been building — that hardcoded path is actually the XBee's FTDI adapter, not a Victron. controller_settings.py opens it thinking it's a Victron, tries to speak VE.Direct to a device that's speaking XBee API frames, and while it does that it races xbee_master_collect.py for every incoming byte. Both processes lose ~90% of their reads. Watching either process's log, this manifests as the XBee master silently missing most incoming SG?/RD frames from Robin nodes, with no error and no obvious clue as to why — the container appears "sort of" alive but throughput is broken.

Fix

Two related changes, both in sparrow/controller_settings.py:

1. DEFAULTS["PORT"] now reads from the VE_DIRECT_PORT env var, falling back to /dev/ttyUSB0 only when the env var isn't set. Fresh installs pick up whatever the setup script writes into sparrow.env.
2. load_config() now forces the env var to win over the saved JSON — an existing deployment with a stale controller_settings.json (already written with /dev/ttyUSB0) no longer gets locked into the wrong port. On restart it re-reads the env var.

Downstream effect on the field Pi we validated this on today: when no Victron cable is present, VE_DIRECT_PORT points at a Victron by-id path that doesn't exist. controller_settings.py tries to open it, serial.Serial raises FileNotFoundError, the existing retry loop catches it and continues without ever touching the XBee's FTDI. xbee_master_collect.py gets exclusive access to the FTDI and receives every frame cleanly.

[Copilot]

Pull request overview

This PR updates sparrow/controller_settings.py to avoid hardcoding a VE.Direct serial device path that can accidentally point at the XBee FTDI adapter on Pi 5 deployments, causing serial read contention and major frame loss. It makes the VE.Direct port selection deployment-driven via VE_DIRECT_PORT, and ensures the runtime always honors that environment value even if a stale JSON config exists.

Changes:

• Set DEFAULTS["PORT"] from VE_DIRECT_PORT (fallback to /dev/ttyUSB0 only if unset).
• Update load_config() so VE_DIRECT_PORT always overrides the saved JSON port value.

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