Session-bind /api/route and /api/overpass

Today both proxies are effectively open to anyone who can set an
Origin header for trails.cool — a third party can use us as a free
BRouter/Overpass relay. Require a live planner session on every call
so abuse traffic costs the scraper a session row (observable,
revocable) before they can issue a single query.

Server:
- New `requireSession(id)` helper — returns the session row or a 401
  Response. Reused by both route handlers.
- `/api/route`: `sessionId` in body is now required and verified;
  rate-limit key always falls back to the session id.
- `/api/overpass`: new `X-Trails-Session` header, verified. Header
  keeps the session out of the request body so the body-keyed cache
  is unaffected.

Client plumbing:
- `useRouting(yjs, sessionId)` — sessionId goes into the /api/route
  body.
- `usePois(sessionId)` → `queryPois(..., sessionId)` → `X-Trails-Session`
  on the proxy call.
- `PlannerMap` + `YjsDebugPanel` gain a `sessionId` prop from
  `SessionView`.

Journal server-to-server:
- Demo-bot and `/api/v1/routes/compute` now POST `/api/sessions` to
  mint a throwaway planner session, then cite it on the forwarded
  `/api/route` call. Planner's `expire-sessions` cron cleans these up
  (7d window) so nothing needs explicit teardown.

Tests:
- 5 unit tests for `requireSession` covering missing / empty /
  non-string / unknown-session / valid-session cases.
- Two integration E2E tests document the 401 for missing session on
  each proxy.
- Pre-existing `/api/route` integration tests updated to mint a
  session first.

Caveat: existing browser tabs lose their /api/route ability until
reload (the old JS doesn't know to send sessionId). Acceptable for
an anonymous planner.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/planner/app/components/YjsDebugPanel.tsx

@@ -76,7 +76,7 @@ const dangerBtnClass =
  * - Actions: reset session, refetch route, become host, kick users
  * - State inspector: awareness, waypoints, route data, doc stats
  */
-export function YjsDebugPanel({ yjs }: { yjs: YjsState }) {
+export function YjsDebugPanel({ yjs, sessionId }: { yjs: YjsState; sessionId: string }) {
   const [visible, setVisible] = useState(() => loadBool("trails:debug", import.meta.env.DEV));
   const [expanded, setExpanded] = useState(() => loadBool("trails:debug:expanded", false));
   const [state, setState] = useState<DebugState | null>(null);
@@ -142,14 +142,14 @@ export function YjsDebugPanel({ yjs }: { yjs: YjsState }) {
     const response = await fetch("/api/route", {
       method: "POST",
       headers: { "Content-Type": "application/json" },
-      body: JSON.stringify({ waypoints, profile }),
+      body: JSON.stringify({ waypoints, profile, sessionId }),
     });
 
     if (response.ok) {
       const geojson = await response.json();
       yjs.routeData.set("geojson", JSON.stringify(geojson));
     }
-  }, [yjs]);
+  }, [yjs, sessionId]);
 
   const kickUser = useCallback(
     (clientId: number) => {