feat(feed): list every transport each drone was heard on

[wnagele]

What

Drone rows in the JSON feed gain an additive rid_sources array listing every transport the drone has been heard on since it entered the tracker, most-recent-first — e.g. ["wifi_beacon", "ble"], or ["wifi_beacon", "ble5", "ble"] once #4 lands. Element 0 always matches the current rid_source; values mirror rid_source. The dashboard Source column renders the full list — current transport at full intensity, previously-heard transports dimmed. The array resets when the drone TTL-evicts and reappears, same as message_count. No schema_version bump.

Why

rid_source is most-recent-wins: with a drone heard on several transports — most transmitters broadcast BLE and Wi-Fi Beacon simultaneously — only the latest one is visible, and there is no way to tell "currently on Wi-Fi Beacon, previously heard on BLE" from the row alone. The granular BLE value from #4 (ble5) makes the field flip even within a single transport family. The array preserves the full history: consumers and operators see which radios are alive and which transports a given drone actually uses, at a glance.

How

• tracker.py: DroneState gains a sources list; _touch_sources() deduplicates and moves the just-heard transport to the front on every update_* call; snapshot() exposes it as the additive rid_sources array. TTL eviction resets it naturally.
• feed_server.py: the dashboard Source column renders through a new transportCell() — element 0 at full intensity, the rest dimmed (.src-old) — falling back to plain rid_source for feeds predating the field.
• FEED.md documents the row; CHANGELOG entry under Added.

Composes with #4: merging that branch into this one verifies cleanly — a drone heard on ble, ble5 and wifi_beacon reports rid_sources: ["wifi_beacon", "ble5", "ble"].