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/events/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'
 
 export async function GET(req: Request) {
   if (!isBearerAuthorized(req)) return unauthorized()
@@ -13,7 +13,7 @@ export async function GET(req: Request) {
   const baseUrl = `http://localhost:${process.env.PORT || '3000'}`
   const r = await fetch(`${baseUrl}/api/calendar?range=${encodeURIComponent(range)}`, {
     cache: 'no-store',
-    headers: { cookie: '' },
+    headers: internalHeaders(),
   }).catch(() => null)
 
   if (!r || !r.ok) return NextResponse.json({ events: [], error: 'unreachable' }, { status: 502 })