fix(planner/brouter): add 30s timeout to outbound fetch

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).

apps/planner/app/lib/http.server.ts

@@ -0,0 +1,15 @@
+// Default timeout for outbound HTTP calls to third-party services
+// (BRouter, Overpass, …). A hung upstream must not stall the request
+// handler indefinitely.
+export const DEFAULT_EXTERNAL_FETCH_TIMEOUT_MS = 30_000;
+
+export function fetchWithTimeout(
+  input: RequestInfo | URL,
+  init: RequestInit = {},
+  timeoutMs: number = DEFAULT_EXTERNAL_FETCH_TIMEOUT_MS,
+): Promise<Response> {
+  const signal = init.signal
+    ? AbortSignal.any([init.signal, AbortSignal.timeout(timeoutMs)])
+    : AbortSignal.timeout(timeoutMs);
+  return fetch(input, { ...init, signal });
+}