Add optional fixed DX cluster spotter coordinates via env vars

[on6zq]

Summary

This PR adds two optional environment variables:

• DX_CLUSTER_SPOTTER_LAT
• DX_CLUSTER_SPOTTER_LON

When both are set, /api/dxcluster/paths uses these fixed coordinates for the DX cluster spotter origin and reports spotterLocSource as "env-fixed".

When unset, existing spotter location behavior remains unchanged.

Motivation

OpenHamClock currently derives the DX cluster spotter location from grid / HamQTH / prefix logic. In some installations, operators want all DX cluster path origins to use a known fixed station location.

Changes

• Added dxClusterSpotterLat and dxClusterSpotterLon to config parsing
• Added a fixed-origin override in /api/dxcluster/paths
• Updated the related configuration/documentation files

Testing

Tested with:

• DX_CLUSTER_SPOTTER_LAT=50.70573
• DX_CLUSTER_SPOTTER_LON=4.39205

Verified /api/dxcluster/paths returns:

• spotterLat: 50.70573
• spotterLon: 4.39205
• spotterLocSource: "env-fixed"

Also verified that when the variables are unset, behavior falls back to the existing automatic spotter location logic.

[on6zq]

Demo on https://openhamclock.on6zq.be/