Support downlink scheduling on all gateway antennas

[vlasebian]

Summary

The Gateway Server only issued uplink tokens for antenna 0 and disabled the downlink path on all other antennas, so multi-board gateways could never receive downlinks on their other antennas. This PR treats all antennas as downlink paths and sends each board its own antenna gain in the LoRa Basics Station router configuration.

References #48

Changes

• Uplinks received on any antenna now carry an uplink token and inherit the gateway's downlink path constraint, instead of DOWNLINK_PATH_CONSTRAINT_NEVER on antennas other than the first.
• Each SX1301_conf object in the LBS router_config now carries the gain of its own board's antenna (aligned index-for-index with the gateway's frequency plans, falling back to the first antenna's gain when fewer antennas are registered), instead of the first antenna's gain applied to all boards.
• Antenna gains are no longer truncated to integers on the LBS path.

Testing

Steps

1. Register a gateway with two frequency plans (e.g. EU_863_870,EU_863_870) and two antennas with different gains (CLI: --antenna.index N --antenna.gain X).
2. Connect a dual-board LoRa Basics Station gateway and verify the router_config contains one SX1301_conf per board, each with its own antenna_gain.
3. Send an uplink via the second board (rctx: 1) and verify the uplink metadata carries an uplink token and the gateway's downlink path constraint.
4. Schedule a class A downlink for that uplink and verify the dnmsg carries rctx: 1.

Results

Unit tests cover the gain alignment and fallback (TestNewConnectionAntennaGains) and per-board gains in the router configuration (TestGetRouterConfigWithMultipleFP/DistinctAntennaGains).

Regressions

• Single-antenna gateways: unchanged behavior (gain of antenna 0, same router_config output; integral gains marshal identically).
• Gateways with fewer registered antennas than frequency plans previously relied on the first antenna's gain for all boards; that fallback is preserved.

Notes for Reviewers

Scheduling downlinks concurrently on different antennas (per-antenna conflict tracking in the scheduler) is a follow-up on this branch; the sub-band duty-cycle accounting intentionally remains per gateway.

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Compatibility: The changes are backwards compatible with existing API, storage, configuration and CLI, according to the compatibility commitments in README.md for the chosen target branch.
• Documentation: Relevant documentation is added or updated.
• Testing: The steps/process to test this feature are clearly explained including testing for regressions.
• Infrastructure: If infrastructural changes (e.g., new RPC, configuration) are needed, a separate issue is created in the infrastructural repositories.
• Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.
• Commits: Commit messages follow guidelines in CONTRIBUTING.md, there are no fixup commits left.