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/lib/server/scraper.ts

@@ -0,0 +1,60 @@
+/**
+ * Backend API helper.
+ *
+ * Centralises the BACKEND_URL constant and provides a thin fetch wrapper that:
+ *   - Resolves paths relative to BACKEND_API_URL.
+ *   - Throws 502 on network errors (unreachable backend).
+ *   - Re-throws SvelteKit `error()` objects so callers can still short-circuit.
+ *   - Passes a RequestInit through verbatim so callers keep full control.
+ *
+ * Import only from server-side modules (`+server.ts`, `*.server.ts`).
+ */
+
+import { error } from '@sveltejs/kit';
+import { env } from '$env/dynamic/private';
+
+export const BACKEND_URL = env.BACKEND_API_URL ?? 'http://localhost:8080';
+
+/**
+ * Fetch a path on the backend, throwing a 502 on network failures.
+ *
+ * The `path` must start with `/` (e.g. `/api/voices`).
+ *
+ * SvelteKit `error()` exceptions are always re-thrown so callers can
+ * short-circuit correctly inside their own catch blocks.
+ */
+export async function backendFetch(path: string, init?: RequestInit): Promise<Response> {
+	try {
+		return await fetch(`${BACKEND_URL}${path}`, init);
+	} catch (e) {
+		// Re-throw SvelteKit HTTP errors so they propagate to the framework.
+		if (e instanceof Error && 'status' in e) throw e;
+		throw error(502, 'Could not reach backend');
+	}
+}
+
+// ─── Response types ───────────────────────────────────────────────────────────
+
+/**
+ * Metadata shape returned inside the 200 response from GET /api/book-preview/{slug}.
+ * Used in both the SSR page load and the API proxy to avoid duplicating the inline type.
+ */
+export interface BookPreviewMeta {
+	slug: string;
+	title: string;
+	author: string;
+	cover: string;
+	status: string;
+	genres: string[];
+	summary: string;
+	total_chapters: number;
+	source_url: string;
+}
+
+/** Full 200 response from GET /api/book-preview/{slug}. */
+export interface BookPreviewResponse {
+	in_lib: boolean;
+	meta: BookPreviewMeta;
+	chapters: { number: number; title: string; date?: string }[];
+}
+