bugfix: proxy /api/* through the SvelteKit container

The compose deploy was unreachable because frontend code reads its
API base from `import.meta.env.VITE_API_BASE` at build time, but the
shipped image baked in the fallback `/api` and never picked up the
`PUBLIC_API_BASE` env var. The browser then hit
http://localhost:3000/api/...which the Node adapter doesn't serve, so
every request 404'd.

Fix the topology at the right layer: hooks.server.ts proxies /api/*
requests through to the backend container over docker's internal
network. The browser only ever talks to :3000, cookies stay
same-origin, and CORS can stay empty.

- frontend/src/hooks.server.ts: new proxy. Reads BACKEND_URL (defaults
  to http://localhost:8080 for ad-hoc node builds). Strips `host` and
  `content-length` so the backend sees the real client request and
  recomputes the length. Sets `duplex: 'half'` for streamed POST
  bodies. GET/HEAD have no body. Non-/api paths fall through to
  SvelteKit normally.
- docker-compose.yml: drop the host port mapping on the backend
  (browser doesn't reach it directly anymore — use `ports:` instead of
  `expose:` if you want curl access). Set BACKEND_URL=http://backend:8080
  on the frontend service. Drop PUBLIC_API_BASE which was unused.
- .env.example: replace PUBLIC_API_BASE with BACKEND_URL, with a note
  on what it does.
- README: explain the new topology in Quick start, update the bot
  curl examples to hit :3000 (since that's the only published port in
  the default deploy), and call out that the TLS terminator only needs
  one upstream now.

Lockstep version bump to 0.9.1.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/hooks.server.ts

@@ -0,0 +1,45 @@
+import type { Handle } from '@sveltejs/kit';
+
+// Reverse-proxy `/api/*` requests through to the backend container.
+//
+// Mangalord's compose runs SvelteKit (this process) on :3000 and axum on
+// :8080. The browser only ever talks to :3000, so cookies stay
+// same-origin and `CORS_ALLOWED_ORIGINS` can stay empty in the default
+// deploy. The backend hostname comes from `BACKEND_URL` (compose wires
+// `http://backend:8080`); for `npm run dev` we fall back to the same
+// localhost target the vite proxy uses, which keeps the dev story
+// consistent even if someone bypasses the vite proxy.
+
+const BACKEND_URL = process.env.BACKEND_URL ?? 'http://localhost:8080';
+
+export const handle: Handle = async ({ event, resolve }) => {
+    if (event.url.pathname.startsWith('/api/')) {
+        const target = `${BACKEND_URL}${event.url.pathname}${event.url.search}`;
+
+        // Strip hop-by-hop headers — `host` would mislead the backend
+        // about the origin, and `content-length` will be recomputed.
+        const headers = new Headers(event.request.headers);
+        headers.delete('host');
+        headers.delete('content-length');
+
+        const init: RequestInit & { duplex?: 'half' } = {
+            method: event.request.method,
+            headers,
+            redirect: 'manual'
+        };
+        if (event.request.method !== 'GET' && event.request.method !== 'HEAD') {
+            init.body = event.request.body;
+            // Node's fetch requires `duplex: 'half'` when streaming a
+            // request body; otherwise the stream is rejected.
+            init.duplex = 'half';
+        }
+
+        const upstream = await fetch(target, init);
+        return new Response(upstream.body, {
+            status: upstream.status,
+            statusText: upstream.statusText,
+            headers: upstream.headers
+        });
+    }
+    return resolve(event);
+};