security: validate Planner callback URL to close an SSRF sink

The session callbackUrl becomes a server-side fetch target in
api.save-to-journal (POSTed with the callback bearer token, and the
journal's response is reflected to the caller). The /new query-param
loader already validated it, but the programmatic POST /api/sessions
entry point — anonymous, since the Planner is stateless — stored it
unvalidated. An attacker could make the Planner backend POST to
arbitrary hosts, including 169.254.169.254 and other internal targets.

- validateFetchUrl now blocks private / loopback / link-local /
  CGNAT / cloud-metadata hosts (IPv4, IPv6, IPv4-mapped) when an
  explicit allowlist isn't set. Gated on NODE_ENV=production && !E2E
  (the requireSecret idiom) so the dev/e2e journal-on-localhost save
  flow is unaffected. An explicit PLANNER_CALLBACK_ALLOWED_HOSTS still
  takes precedence and remains the full-closure control (it also stops
  DNS-name-to-private rebinding, which literal blocking does not).
- POST /api/sessions now validates callbackUrl exactly as /new does.
- api.save-to-journal re-validates immediately before the fetch
  (defense in depth: covers sessions persisted before this change and
  narrows the create→save rebinding window).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

apps/planner/app/routes/api.sessions.ts

@@ -4,6 +4,7 @@ import { createSession, listSessions } from "~/lib/sessions";
 import { parseGpxAsync, extractWaypoints } from "@trails-cool/gpx";
 import { withDb } from "@trails-cool/db";
 import type { Waypoint } from "@trails-cool/types";
+import { validateFetchUrl, getCallbackAllowedHosts } from "~/lib/url-validation.server";
 
 export async function action({ request }: Route.ActionArgs) {
   if (request.method !== "POST") {
@@ -17,6 +18,20 @@ export async function action({ request }: Route.ActionArgs) {
     gpx?: string;
   };
 
+  // callbackUrl becomes a server-side fetch target on save-to-journal,
+  // so an unvalidated value here is an SSRF sink. The /new loader
+  // already validates the query-param form; this is the programmatic
+  // JSON entry point and must do the same.
+  if (callbackUrl !== undefined) {
+    if (typeof callbackUrl !== "string") {
+      return data({ error: "callbackUrl must be a string" }, { status: 400 });
+    }
+    const v = validateFetchUrl(callbackUrl, { allowedHosts: getCallbackAllowedHosts() });
+    if (!v.ok) {
+      return data({ error: `Invalid callback URL: ${v.reason}` }, { status: 400 });
+    }
+  }
+
   return withDb(async () => {
     const session = await createSession({ callbackUrl, callbackToken });