Bug: Tracking external-feed polling has no in-flight cancel, no per-fetch timeout, and setState fires on unmounted component

[kevinelliott]

Summary

pollExternalFeeds in client/src/pages/Tracking.tsx (lines 221–304) runs on a setInterval(..., 2000) and does sequential fetches across all enabled feeds. There is no AbortController, no per-fetch timeout, and no "component still mounted" guard.

Impact

Two concrete failure modes:

1. Overlapping in-flight requests. If one feed URL (or the third-party proxy from Bug: Tracking page external feed loader uses a hard-coded third-party CORS proxy and reads unvalidated localStorage JSON #24) hangs beyond 2 seconds, the next setInterval tick starts a fresh sweep on top of the previous one. Requests accumulate faster than they resolve; the manager and/or the third-party proxy see a slowly growing pileup.

2. setState on unmounted component. After the user navigates away from the Tracking page, the interval is cleared, but any in-flight fetches keep running. When they resolve they call setExternalVehicles on a dead component (React logs a warning; over time this leaks memory too).

Suggested fix

• Wrap each fetch with AbortController + a per-request timeout (e.g. 5s).
• Store the current sweep's controllers in a ref so the effect cleanup can .abort() them.
• Track an alive ref set to true on mount, false on unmount, and short-circuit setExternalVehicles when !alive.current.

Related but distinct from #24 (which is about the third-party CORS proxy and unvalidated localStorage JSON) — this one is specifically about the polling mechanics.