fix: increase body size limit to 500MB for audio uploads

Default SvelteKit node adapter body limit is 512KB — too small for
audio recordings (30s ~556KB, longer recordings much larger).
Set bodySize: 500MB in adapter config. Also set BODY_SIZE_LIMIT env
in production compose .env as belt-and-suspenders.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

svelte.config.js

@@ -6,7 +6,7 @@ const config = {
 		runes: ({ filename }) => (filename.split(/[/\\]/).includes('node_modules') ? undefined : true)
 	},
 	kit: {
-		adapter: adapter({ out: 'build' }),
+		adapter: adapter({ out: 'build', bodySize: 500 * 1024 * 1024 }), // 500 MB for audio uploads
 		// CSRF origin check disabled: this app uses no cookie-based session auth,
 		// and the Web Share Target POST legitimately arrives from external origins.
 		csrf: { checkOrigin: false }