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/lib/server/db/index.ts

@@ -1,9 +1,9 @@
-import { env } from '$env/dynamic/private';
+import { DB_URL } from '$app/env/private';
 import { relations } from './relations';
 import { drizzle } from 'drizzle-orm/postgres-js';
 import postgres from 'postgres';
 
-const client = postgres(env.DB_URL, { prepare: false });
+const client = postgres(DB_URL, { prepare: false });
 
 export const db = drizzle({
 	relations,

src/lib/server/db/schema/index.ts

@@ -1,5 +1,5 @@
 // One file per domain. Everything is re-exported here so the rest of the app
-// (and drizzle-kit / relations) only ever imports `$lib/server/db/schema`.
+// (and drizzle-kit / relations) only ever imports `#lib/server/db/schema/index.js`.
 export * from './auth';
 export * from './blog';
 export * from './progress';