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/blog/[slug]/+page.server.ts

@@ -1,7 +1,7 @@
-import { db } from '$lib/server/db';
-import { blogArticles } from '$lib/server/db/schema';
-import { isAdmin } from '$lib/server/auth';
-import { renderMarkdown } from '$lib/server/markdown';
+import { db } from '#lib/server/db/index.js';
+import { blogArticles } from '#lib/server/db/schema/index.js';
+import { isAdmin } from '#lib/server/auth.js';
+import { renderMarkdown } from '#lib/server/markdown.js';
 import { eq } from 'drizzle-orm';
 
 import type { PageServerLoad } from './$types.js';
@@ -14,14 +14,14 @@ export const load: PageServerLoad = async (event) => {
 		.where(eq(blogArticles.slug, event.params.slug));
 
 	if (!article) {
-		return error(404, { message: 'Article not found' });
+		return error(404, 'Article not found');
 	}
 
 	const admin = isAdmin(event);
 
 	if (article.status !== 'published') {
 		if (!admin) {
-			return error(404, { message: 'Article not found' });
+			return error(404, 'Article not found');
 		}
 		// Drafts are only ever viewed/edited through the editor.
 		return redirect(303, `/blog/${article.slug}/edit`);