Feat v0.5.3 chat polish + integration testing

Conversation search (db.query search_like, chat-core /search endpoint,
sidebar search UI), message pagination polish (scroll preservation,
loading spinner), workflow-chat integration package, and multi-user
E2E test infrastructure.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ci/e2e-nginx.conf

@@ -0,0 +1,39 @@
+events {
+  worker_connections 128;
+}
+
+http {
+  upstream switchboard {
+    server switchboard-1:80;
+    server switchboard-2:80;
+  }
+
+  # WebSocket upgrade map
+  map $http_upgrade $connection_upgrade {
+    default upgrade;
+    ''      close;
+  }
+
+  server {
+    listen 80;
+
+    location / {
+      proxy_pass http://switchboard;
+      proxy_set_header Host $host;
+      proxy_set_header X-Real-IP $remote_addr;
+      proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+      proxy_set_header X-Forwarded-Proto $scheme;
+    }
+
+    # WebSocket endpoint
+    location /ws {
+      proxy_pass http://switchboard;
+      proxy_http_version 1.1;
+      proxy_set_header Upgrade $http_upgrade;
+      proxy_set_header Connection $connection_upgrade;
+      proxy_set_header Host $host;
+      proxy_set_header X-Real-IP $remote_addr;
+      proxy_read_timeout 86400s;
+    }
+  }
+}