Fix port conflict: separate Next.js (3001) from Go API (PORT)

- Next.js admin now runs on internal port 3001
- Go API uses Dokku's PORT environment variable (5000)
- Updated admin proxy default to localhost:3001
- Added /_next/* static asset proxy route

This fixes the "address already in use" error when deploying to Dokku
where both services were trying to bind to port 3000.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

start.sh

@@ -11,8 +11,12 @@ if [ ! -f server.js ]; then
   exit 1
 fi
 
-echo "Starting Next.js admin on :3000..."
-HOSTNAME="0.0.0.0" PORT=3000 NODE_ENV=production node server.js &
+# Save the main PORT for Go API (Dokku sets this)
+API_PORT="${PORT:-5000}"
+
+# Start Next.js admin on internal port 3001
+echo "Starting Next.js admin on :3001..."
+HOSTNAME="0.0.0.0" PORT=3001 NODE_ENV=production node server.js &
 NODE_PID=$!
 
 # Give Next.js a moment to boot
@@ -24,5 +28,7 @@ if ! kill -0 "$NODE_PID" 2>/dev/null; then
   exit 1
 fi
 
-echo "Starting Go API on :${PORT:-5000}..."
+# Start Go API on main PORT that Dokku expects
+echo "Starting Go API on :${API_PORT}..."
+export PORT="${API_PORT}"
 exec /app/api