fix(voice/tools): use x-voice-internal header for loopback fetches

Tool endpoints (events, notes, transport, weather) call other /api/*
routes via loopback (http://localhost:3000). Those routes are
middleware-protected — cookie-less loopbacks were getting 401, which
surfaced to the voice agent as get_today_events → tool_http_502.

Add internal header bypass: middleware lets the request through when
x-voice-internal matches VOICE_API_KEY. Only our own tool endpoints
use this header, from inside the same container, so the blast radius
is limited to loopback traffic.

- middleware.ts: check x-voice-internal before cookie
- lib/voice-tools.ts: internalHeaders() helper
- app/api/voice/tools/{weather,transport,events,notes}: use it

app/api/voice/tools/transport/route.ts

@@ -2,7 +2,7 @@ export const dynamic = 'force-dynamic'
 export const runtime = 'nodejs'
 
 import { NextResponse } from 'next/server'
-import { isBearerAuthorized, unauthorized } from '@/lib/voice-tools'
+import { isBearerAuthorized, unauthorized, internalHeaders } from '@/lib/voice-tools'
 
 // Hardcoded for now — same as TransportWidget. Future: read from /data/tablet-config.json.
 const STOPS: Record<string, { id: string; name: string; direction: string }> = {
@@ -32,7 +32,7 @@ export async function GET(req: Request) {
     dirsToQuery.map(async (d) => {
       const r = await fetch(`${baseUrl}/api/transport?stopId=${d.id}`, {
         cache: 'no-store',
-        headers: { cookie: '' },
+        headers: internalHeaders(),
       }).catch(() => null)
       if (!r || !r.ok) return { direction: d.direction, stop_id: d.id, arrivals: [] }
       const j = await r.json()