fix: extract softAuth to shared module and apply to icon_sets router

Move the softAuth middleware from devices.js into a new shared
lib/api/soft_auth.js module. Apply router.use(softAuth) and
router.use(errorMiddleware) to icon_sets.js so that POST/DELETE
owner-only routes return 401 (not 500) when no auth is present.

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

lib/api/soft_auth.js

@@ -0,0 +1,25 @@
+// lib/api/soft_auth.js — shared middleware
+// Soft auth: identifies the actor if auth is present but never blocks the request.
+// Owner-only sub-routes enforce 401/403 via requireOwner.
+import * as agents from '../db/repos/agents.js';
+import { timingSafeStrEqual } from '../auth/safe_compare.js';
+import { accessOwnerEmail } from '../auth/cf_access.js';
+
+export async function softAuth(req, _res, next) {
+  try {
+    const cfEmail = await accessOwnerEmail(req);
+    if (cfEmail) { req.actor = { kind: 'user', id: null }; return next(); }
+    const auth = req.headers.authorization || '';
+    const [scheme, token] = auth.split(' ');
+    if (scheme === 'Bearer' && token) {
+      if (process.env.OWNER_TOKEN && timingSafeStrEqual(token, process.env.OWNER_TOKEN)) {
+        req.actor = { kind: 'user', id: null }; return next();
+      }
+      try {
+        const agent = await agents.verifyToken(token);
+        if (agent) req.actor = { kind: 'agent', id: agent.id, capabilities: agent.capabilities || {}, scopes: agent.scopes || {} };
+      } catch { /* ignore */ }
+    }
+  } catch { /* ignore */ }
+  next();
+}