step 5 (continued): prune stale root files, drop frontend split

Delete Dockerfile.frontend and docker-entrypoint-fe.sh (single-image
decision per ROADMAP). Remove gutted /chat, /notes, /projects routes
from nginx.conf. Fix Chat Switchboard branding in entrypoint and
.env.example. Update DB names to switchboard_core, remove dead
Extraction config.

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

docker-entrypoint.sh

@@ -1,6 +1,6 @@
 #!/bin/bash
 # ============================================
-# Chat Switchboard - Backend Launcher
+# Switchboard Core - Backend Launcher
 # ============================================
 # Runs as an nginx entrypoint.d hook.
 # Starts the Go backend in the background
@@ -9,7 +9,7 @@
 
 set -e
 
-echo "🔀 Starting Chat Switchboard backend..."
+echo "🔀 Starting Switchboard Core backend..."
 
 # Launch Go backend in background (from /app so migrations are found)
 cd /app