feat(build): inject build time and version into health endpoint

Vite define injects __BUILD_TIME__ and __BUILD_VERSION__ at build time
so /health returns the exact timestamp and package version of the running
build — useful for verifying deploys without guessing what changed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app.d.ts

@@ -10,6 +10,9 @@ declare namespace App {
     // interface Platform {}
 }
 
+declare const __BUILD_TIME__: string;
+declare const __BUILD_VERSION__: string;
+
 declare global {
     namespace App {
         interface Platform {}

src/routes/health/+server.ts

@@ -1,16 +1,13 @@
 import { json } from '@sveltejs/kit';
 import type { RequestHandler } from './$types';
 
-/**
- * Health Check Endpoint
- * Used by Docker and Nginx to verify the service is running.
- */
 export const GET: RequestHandler = async () => {
     return json(
         {
             status: 'healthy',
-            timestamp: new Date().toISOString(),
             service: 'aether-app-sveltekit',
+            version: __BUILD_VERSION__,
+            build_time: __BUILD_TIME__,
             node_env: process.env.NODE_ENV || 'development'
         },
         {