chore: migrate to v3, Doppler secrets, clean up legacy code

- Remove all pre-v3 code: scraper, ui-v2, backend v1, ios v1+v2, legacy CI workflows
- Flatten v3/ contents to repo root
- Add Doppler secrets management (project=libnovel, config=prd)
- Add justfile with doppler run wrappers for all docker compose commands
- Strip hardcoded env fallbacks from docker-compose.yml
- Add minimal README.md
- Clean up .gitignore

ui/src/hooks.client.ts

@@ -0,0 +1,13 @@
+import * as Sentry from '@sentry/sveltekit';
+import { env } from '$env/dynamic/public';
+
+// Sentry / GlitchTip client-side error tracking.
+// No-op when PUBLIC_GLITCHTIP_DSN is unset (e.g. local dev).
+if (env.PUBLIC_GLITCHTIP_DSN) {
+	Sentry.init({
+		dsn: env.PUBLIC_GLITCHTIP_DSN,
+		tracesSampleRate: 0.1
+	});
+}
+
+export const handleError = Sentry.handleErrorWithSentry();