feat: upgrade to SvelteKit 3 and adapter-bun

Run the sveltekit-3 migration and finish its manual tasks:

- Move kit config into the sveltekit() Vite plugin; drop svelte.config.js
- Replace $lib with #lib subpath imports
- Declare env vars in src/env.ts and import them from $app/env/{private,public}
- Consolidate param matchers into src/params.ts
- Use error(status, message), Response.json, refreshAll and goto reset/shallow
- Fall back to a full page load when goto rejects a login return path

Switch from adapter-node to adapter-bun: the build script runs Vite under
Bun (the adapter requires it) and the image starts with `bun ./build`.
Runtime env names (PORT, BODY_SIZE_LIMIT, ADDRESS_HEADER, ...) are unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/routes/sitemap.xml/+server.ts

@@ -1,8 +1,8 @@
-import { db } from '$lib/server/db';
-import { blogArticles } from '$lib/server/db/schema';
-import { SITE_ORIGIN } from '$lib/seo';
-import { races } from '$lib/derby/races';
-import { COURSES } from '$lib/language-learner/courses';
+import { db } from '#lib/server/db/index.js';
+import { blogArticles } from '#lib/server/db/schema/index.js';
+import { SITE_ORIGIN } from '#lib/seo.js';
+import { races } from '#lib/derby/races.js';
+import { COURSES } from '#lib/language-learner/courses/index.js';
 import { eq } from 'drizzle-orm';
 import type { RequestHandler } from './$types';