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>

vite.config.ts

@@ -1,9 +1,16 @@
 import { defineConfig } from 'vite';
 import { sveltekit } from '@sveltejs/kit/vite';
 import tailwindcss from '@tailwindcss/vite';
+import { readFileSync } from 'fs';
+
+const pkg = JSON.parse(readFileSync('./package.json', 'utf-8'));
 
 export default defineConfig({
     cacheDir: 'node_modules/.vite',
+    define: {
+        __BUILD_TIME__: JSON.stringify(new Date().toISOString()),
+        __BUILD_VERSION__: JSON.stringify(pkg.version)
+    },
     plugins: [
         tailwindcss(),
         sveltekit() // <-- Must come after Tailwind