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/app.d.ts

@@ -13,6 +13,14 @@ declare global {
 		// interface PageState {}
 		// interface Platform {}
 	}
+
+	// Umami analytics — injected by the script tag in +layout.svelte when
+	// PUBLIC_UMAMI_WEBSITE_ID is set. Optional so calls are no-ops in dev.
+	interface Window {
+		umami?: {
+			track(event: string, data?: Record<string, unknown>): void;
+		};
+	}
 }
 
 export {};