fix(auth): constant-time owner-token comparison

Owner bearer token was compared with === / !==, which short-circuits on the
first differing byte and leaks token length+prefix via response timing
(security-sweep-2026-06-01.md). New timingSafeStrEqual (crypto.timingSafeEqual
with a length pre-check so it never throws on length mismatch); wired into both
owner.js and agent_auth.js.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/api/middleware/agent_auth.js

@@ -1,4 +1,5 @@
 import * as agents from '../../db/repos/agents.js';
+import { timingSafeStrEqual } from '../../auth/safe_compare.js';
 
 export async function agentOrOwner(req, res, next) {
   const expectedOwner = process.env.OWNER_TOKEN;
@@ -12,7 +13,7 @@ export async function agentOrOwner(req, res, next) {
   if (scheme !== 'Bearer' || !token) {
     return res.status(401).json({ error: { code: 'unauthorized', message: 'missing bearer token' } });
   }
-  if (token === expectedOwner) {
+  if (timingSafeStrEqual(token, expectedOwner)) {
     req.actor = { kind: 'user', id: null };
     return next();
   }