enroll devices against the headscale server the owner picked

/api/vpn/enroll minted pre-auth keys itself, from HEADSCALE_URL, HEADSCALE_API_KEY
and HEADSCALE_USER in the host env. Three globals describe one server; Officer keeps
a registry of many in headscale_servers with one active, so the env could contradict
the server the owner had selected — and HEADSCALE_USER filed every joining device
under the same name on all of them.

The two credential vars had already been removed from the environment and nothing
noticed: the route checks `if (!base || !apiKey)` first, so it had been answering
503 to every enrollment attempt, silently. HEADSCALE_USER was read but never reached.

Enrollment moves into the sidecar that owns the registry and acts on the active
server. The owning user is resolved rather than hardcoded: an explicit userId wins,
one user on the server needs no choice, several is a 409 listing them instead of a
silent guess. The platform route keeps its path and response shape — both are a
contract with enrollVpn() in the mobile core — and is now a bare forward holding no
Headscale URL, key or user name.

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

src/servers/sidecar/headscale/enroll.ts

@@ -0,0 +1,96 @@
+import type { OfficerContext } from './routes';
+import type { OfficerUser } from './normalize';
+import { getActiveHeadscaleCredentials } from 'officerdb';
+import { badRequest, methodNotAllowed, readJson } from './routes';
+import { createClient, type HeadscaleClient } from './client';
+import { arrayField, toUser } from './normalize';
+
+// Device enrolment — POST /_officer/enroll. The mobile app's one-tap join: it turns an authenticated
+// Officer session into a short-lived, single-use pre-auth key, so nobody pastes a key by hand.
+//
+// THIS USED TO LIVE IN THE PLATFORM. `src/servers/api/vpn/router.ts` read HEADSCALE_URL, HEADSCALE_API_KEY
+// and HEADSCALE_USER straight from the host env — three globals that could only ever describe ONE server,
+// while this sidecar already kept a registry of many. Worse, the two credential vars were removed at some
+// point and nobody noticed: the route had been answering 503 to every enrolment attempt, because it checks
+// those two before it gets anywhere near the user name. Enrolment acts on the ACTIVE registered server now,
+// like every other domain route here, and the platform holds no Headscale credentials at all.
+//
+// The response shape `{controlUrl, authKey}` is a CONTRACT: enrollVpn() in the mobile core
+// (monorepo-mobile/packages/core/src/services/officer-net.ts) destructures exactly those two fields and
+// feeds them to configure()/loginWithAuthKey(). Extra fields are safe; renaming those two is not.
+
+/** Short by design: the key is redeemed seconds after it is issued, and a leaked one should die quickly. */
+const KEY_TTL_MS = 10 * 60_000;
+
+/**
+ * Which Headscale user the joining device is filed under.
+ *
+ * An explicit `userId` wins. Otherwise the choice is only made when it is UNAMBIGUOUS — one user on the
+ * server means there is nothing to choose. Several means the caller has to say, because picking silently
+ * files someone's phone under the wrong owner and the mistake stays invisible until somebody audits the
+ * tailnet. The old env var picked one name for every server at once, which is precisely that bug.
+ */
+async function resolveOwner(client: HeadscaleClient, ctx: OfficerContext): Promise<OfficerUser | Response> {
+  const body = await readJson(ctx.req);
+  const requested = typeof body?.userId === 'string' ? body.userId.trim() : '';
+
+  const listed = await client.call('/api/v1/user');
+  const users = arrayField(listed, 'users')
+    .map(toUser)
+    .filter((u): u is OfficerUser => !!u);
+
+  if (requested) {
+    const match = users.find((u) => u.id === requested);
+    return match ?? badRequest(`no Headscale user with id ${requested} on the active server`);
+  }
+
+  if (users.length === 1) return users[0]!;
+
+  if (users.length === 0) {
+    return Response.json(
+      { error: 'the active Headscale server has no users — create one before enrolling a device', code: 'no_users' },
+      { status: 409 },
+    );
+  }
+
+  return Response.json(
+    {
+      error: 'the active Headscale server has several users — pass userId to say which one owns this device',
+      code: 'ambiguous_user',
+      users: users.map((u) => ({ id: u.id, name: u.name })),
+    },
+    { status: 409 },
+  );
+}
+
+export async function handleEnrollRoute(ctx: OfficerContext, segments: string[]): Promise<Response | null> {
+  if (segments.length > 0) return null;
+  if (ctx.req.method !== 'POST') return methodNotAllowed();
+
+  // Not activeClient(): the control URL goes back to the device, and only the credentials carry it.
+  const creds = await getActiveHeadscaleCredentials(ctx.userId);
+  if (!creds) {
+    return Response.json({ error: 'no active Headscale server', code: 'no_active_server' }, { status: 409 });
+  }
+
+  const client = createClient(creds);
+
+  const owner = await resolveOwner(client, ctx);
+  if (owner instanceof Response) return owner;
+
+  const created = await client.call<{ preAuthKey?: { key?: string } }>('/api/v1/preauthkey', {
+    method: 'POST',
+    body: {
+      user: owner.id,
+      reusable: false, // one key, one device
+      ephemeral: false, // the node stays registered after it disconnects
+      expiration: new Date(Date.now() + KEY_TTL_MS).toISOString(), // RFC3339
+    },
+  });
+
+  const authKey = created.preAuthKey?.key;
+  if (!authKey) return Response.json({ error: 'headscale returned no key' }, { status: 502 });
+
+  // `server` and `user` are advisory — for a UI that wants to say what the device just joined.
+  return Response.json({ controlUrl: creds.url, authKey, server: creds.name, user: owner.name });
+}