feat(v3): add v3 stack — backend rewrite, renamed env vars, docs

- New Go backend binary (backend + runner) replacing old scraper/
- Rename SCRAPER_API_URL → BACKEND_API_URL in UI env and docker-compose
- Rename scraperFetch → backendFetch across all 19 UI server files
- Remove SCRAPER_PROXY env var and proxy transport from browser.Config
- Add Meilisearch, Valkey, Caddy to docker-compose
- Add docs/: api-endpoints.md, request-flow.mermaid.md, data-flow.mermaid.md

v3/ui/src/routes/api/users/[username]/+server.ts

@@ -0,0 +1,46 @@
+import { json, error } from '@sveltejs/kit';
+import type { RequestHandler } from './$types';
+import { getPublicProfile, getSubscription } from '$lib/server/pocketbase';
+import { presignAvatarUrl } from '$lib/server/minio';
+import { log } from '$lib/server/logger';
+
+/**
+ * GET /api/users/[username]
+ * Returns public profile info + whether the current user is subscribed.
+ */
+export const GET: RequestHandler = async ({ params, locals }) => {
+	const { username } = params;
+
+	try {
+		const profile = await getPublicProfile(username);
+		if (!profile) error(404, `User "${username}" not found`);
+
+		// Resolve avatar presigned URL if set
+		let avatarUrl: string | null = null;
+		if (profile.avatar_url) {
+			avatarUrl = await presignAvatarUrl(profile.id).catch(() => null);
+		}
+
+		// Is the current logged-in user subscribed?
+		let isSubscribed = false;
+		if (locals.user && locals.user.id !== profile.id) {
+			const sub = await getSubscription(locals.user.id, profile.id).catch(() => null);
+			isSubscribed = !!sub;
+		}
+
+		return json({
+			id: profile.id,
+			username: profile.username,
+			avatarUrl,
+			created: profile.created,
+			followerCount: profile.followerCount,
+			followingCount: profile.followingCount,
+			isSubscribed,
+			isSelf: locals.user?.id === profile.id
+		});
+	} catch (e) {
+		if ((e as { status?: number }).status === 404) throw e;
+		log.error('api/users', 'failed to load profile', { username, err: String(e) });
+		error(500, 'Failed to load profile');
+	}
+};

v3/ui/src/routes/api/users/[username]/library/+server.ts

@@ -0,0 +1,43 @@
+import { json, error } from '@sveltejs/kit';
+import type { RequestHandler } from './$types';
+import {
+	getUserByUsername,
+	getUserPublicLibrary,
+	getUserCurrentlyReading
+} from '$lib/server/pocketbase';
+import { log } from '$lib/server/logger';
+
+/**
+ * GET /api/users/[username]/library
+ * Returns the public library + currently-reading list for a user.
+ * Does not require authentication — all data is public.
+ */
+export const GET: RequestHandler = async ({ params }) => {
+	const { username } = params;
+
+	const user = await getUserByUsername(username).catch(() => null);
+	if (!user) error(404, `User "${username}" not found`);
+
+	try {
+		const [currentlyReading, library] = await Promise.all([
+			getUserCurrentlyReading(user.id),
+			getUserPublicLibrary(user.id)
+		]);
+
+		return json({
+			currently_reading: currentlyReading.map((item) => ({
+				book: item.book,
+				last_chapter: item.chapter,
+				saved: false
+			})),
+			library: library.map((item) => ({
+				book: item.book,
+				last_chapter: item.chapter,
+				saved: item.saved
+			}))
+		});
+	} catch (e) {
+		log.error('api/users/library', 'failed to load library', { username, err: String(e) });
+		error(500, 'Failed to load library');
+	}
+};

v3/ui/src/routes/api/users/[username]/subscribe/+server.ts

@@ -0,0 +1,48 @@
+import { json, error } from '@sveltejs/kit';
+import type { RequestHandler } from './$types';
+import {
+	getUserByUsername,
+	subscribe,
+	unsubscribe,
+	getSubscription
+} from '$lib/server/pocketbase';
+import { log } from '$lib/server/logger';
+
+/**
+ * POST /api/users/[username]/subscribe  — subscribe to a user
+ * DELETE /api/users/[username]/subscribe — unsubscribe
+ * Requires authentication.
+ */
+export const POST: RequestHandler = async ({ params, locals }) => {
+	if (!locals.user) error(401, 'Login required');
+
+	const { username } = params;
+	const target = await getUserByUsername(username).catch(() => null);
+	if (!target) error(404, `User "${username}" not found`);
+	if (locals.user.id === target.id) error(400, 'Cannot subscribe to yourself');
+
+	try {
+		await subscribe(locals.user.id, target.id);
+		const sub = await getSubscription(locals.user.id, target.id);
+		return json({ subscribed: true, subId: sub?.id ?? null });
+	} catch (e) {
+		log.error('api/users/subscribe', 'subscribe failed', { username, err: String(e) });
+		error(500, 'Failed to subscribe');
+	}
+};
+
+export const DELETE: RequestHandler = async ({ params, locals }) => {
+	if (!locals.user) error(401, 'Login required');
+
+	const { username } = params;
+	const target = await getUserByUsername(username).catch(() => null);
+	if (!target) error(404, `User "${username}" not found`);
+
+	try {
+		await unsubscribe(locals.user.id, target.id);
+		return json({ subscribed: false });
+	} catch (e) {
+		log.error('api/users/subscribe', 'unsubscribe failed', { username, err: String(e) });
+		error(500, 'Failed to unsubscribe');
+	}
+};