Mirrors journal PR #5. `fetchSegment()` and `computeSegmentGpx()` in
lib/brouter.ts called `fetch()` with no AbortSignal — a hung or slow
BRouter would stall the request handler indefinitely.
New `lib/http.server.ts::fetchWithTimeout()` (same as the journal's
helper) wraps fetch with `AbortSignal.timeout(30_000)`, composable
with a caller-supplied signal via `AbortSignal.any()`.
Tests: lib/http.server.test.ts (2 cases — timeout abort, happy path).
