fix(mam-api): SESSION_SECRET boot guard + cleaner CORS rejection

Code-review feedback:
- Hard-fail boot when AUTH_ENABLED=true and SESSION_SECRET is unset, so
  express-session can't silently use an in-memory random secret that
  invalidates sessions on restart and breaks multi-node clusters.
- CORS rejection now returns cb(null, false) instead of cb(new Error)
  so misconfigured origins surface as clean CORS errors in the browser
  instead of HTTP 500s. Log a warn line for operator visibility.
- pruneSessionInterval units comment.

services/mam-api/src/index.js

@@ -46,7 +46,10 @@ app.use(cors({
     // No Origin header (same-origin or curl) — allow.
     if (!origin) return cb(null, true);
     if (allowedOrigins.length === 0 || allowedOrigins.includes(origin)) return cb(null, true);
-    return cb(new Error('CORS: origin not allowed: ' + origin));
+    // Reject cleanly: omit the Allow-Origin header so the browser surfaces
+    // a real CORS error instead of a 500 from a thrown Error in the callback.
+    console.warn('[cors] rejected origin:', origin);
+    return cb(null, false);
   },
   credentials: true,
 }));
@@ -55,9 +58,17 @@ app.use(express.json({ limit: '50mb' }));
 // Trust the reverse proxy only when explicitly told to (production HTTPS).
 if (process.env.TRUST_PROXY === 'true') app.set('trust proxy', 1);
 
+// Hard-fail when production-mode auth has no stable session secret. Without
+// this, express-session falls back to an in-memory random secret which
+// invalidates every session on restart and breaks multi-node deployments.
+if (process.env.AUTH_ENABLED === 'true' && !process.env.SESSION_SECRET) {
+  console.error('[fatal] SESSION_SECRET is required when AUTH_ENABLED=true');
+  process.exit(1);
+}
+
 // Session — actually wired this time. See specs/2026-05-27-auth-system-design.md.
 app.use(session({
-  store: new PgStore({ pool, tableName: 'sessions', pruneSessionInterval: 60 * 15 }),
+  store: new PgStore({ pool, tableName: 'sessions', pruneSessionInterval: 60 * 15 /* seconds = 15 min */ }),
   secret: process.env.SESSION_SECRET,
   name:   'dragonflight.sid',
   cookie: {