chore: full hop-by-hop header strip and 60s timeout on /api/* proxy

The SvelteKit proxy was only stripping host + content-length; the rest
of RFC 7230 §6.1 (connection, keep-alive, proxy-authenticate,
proxy-authorization, te, trailer, transfer-encoding, upgrade) leaked
through to axum. Axum doesn't emit them so the impact is theoretical,
but the proxy should be RFC-conformant. Also adds an AbortController
with a configurable 60s timeout (BACKEND_PROXY_TIMEOUT_MS) so a
wedged backend can't hang the browser request indefinitely — failures
surface as the standard 502 upstream_unavailable envelope.

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

.env.example

@@ -51,3 +51,8 @@ MAX_FILE_BYTES=20971520
 # internal docker network. Override only if you're running the
 # frontend container against a backend somewhere else.
 BACKEND_URL=http://backend:8080
+# Per-request wall-clock cap for the /api/* reverse proxy (milliseconds).
+# Default 300000 (5 min) covers a typical 200 MiB chapter upload over
+# 25 Mbps; raise for users on slower upstream links or lower if a
+# tighter front proxy already bounds the request lifetime.
+BACKEND_PROXY_TIMEOUT_MS=300000

frontend/src/hooks.server.test.ts

@@ -118,4 +118,77 @@ describe('hooks.server proxy', () => {
         expect(body.error.code).toBe('upstream_unavailable');
         expect(errSpy).toHaveBeenCalled();
     });
+
+    it('strips every hop-by-hop header listed in RFC 7230 §6.1', async () => {
+        // Defence in depth: axum doesn't emit these, but a future
+        // middleware that did would otherwise leak per-connection
+        // state across the proxy boundary.
+        fetchSpy.mockResolvedValueOnce(new Response('[]', { status: 200 }));
+        const resolve = vi.fn();
+        await handle({
+            event: makeEvent('/api/v1/health', {
+                headers: {
+                    host: 'app.example.com',
+                    'content-length': '0',
+                    connection: 'keep-alive',
+                    'keep-alive': 'timeout=5',
+                    'proxy-authenticate': 'Basic realm=x',
+                    'proxy-authorization': 'Basic xyz',
+                    te: 'trailers',
+                    trailer: 'Expires',
+                    'transfer-encoding': 'chunked',
+                    upgrade: 'websocket',
+                    // A non-hop-by-hop header to ensure non-targets
+                    // aren't accidentally stripped.
+                    'x-custom': 'pass-through'
+                }
+            }),
+            resolve
+        });
+        const init = fetchSpy.mock.calls[0][1] as RequestInit;
+        const headers = init.headers as Headers;
+        for (const h of [
+            'host',
+            'content-length',
+            'connection',
+            'keep-alive',
+            'proxy-authenticate',
+            'proxy-authorization',
+            'te',
+            'trailer',
+            'transfer-encoding',
+            'upgrade'
+        ]) {
+            expect(headers.get(h), `${h} should be stripped`).toBeNull();
+        }
+        expect(headers.get('x-custom')).toBe('pass-through');
+    });
+
+    it('aborts and returns 502 when the upstream stalls past the timeout', async () => {
+        const errSpy = vi.spyOn(console, 'error').mockImplementation(() => {});
+        // Simulate an aborted fetch (AbortController.abort() raises a
+        // DOMException with name 'AbortError' on Node's fetch). The
+        // handler should treat it as the same upstream_unavailable
+        // 502 it uses for any other network failure.
+        const abortErr = new DOMException('aborted', 'AbortError');
+        fetchSpy.mockRejectedValueOnce(abortErr);
+
+        const resolve = vi.fn();
+        const resp = await handle({ event: makeEvent('/api/v1/slow'), resolve });
+        expect(resp.status).toBe(502);
+        const body = await resp.json();
+        expect(body.error.code).toBe('upstream_unavailable');
+        expect(errSpy).toHaveBeenCalled();
+    });
+
+    it('attaches an AbortSignal to the upstream fetch so it can time out', async () => {
+        fetchSpy.mockResolvedValueOnce(new Response('[]', { status: 200 }));
+        const resolve = vi.fn();
+        await handle({ event: makeEvent('/api/v1/health'), resolve });
+        const init = fetchSpy.mock.calls[0][1] as RequestInit;
+        expect(init.signal).toBeInstanceOf(AbortSignal);
+        // The signal hasn't fired (handler returned in time), but its
+        // presence is the contract this test is pinning.
+        expect(init.signal?.aborted).toBe(false);
+    });
 });

frontend/src/hooks.server.ts

@@ -12,20 +12,66 @@ import type { Handle } from '@sveltejs/kit';
 
 const BACKEND_URL = process.env.BACKEND_URL ?? 'http://localhost:8080';
 
+/**
+ * Hop-by-hop headers per RFC 7230 §6.1. These are scoped to a single
+ * transport-level connection and must not be forwarded by a proxy.
+ * Plus `host` and `content-length`: `host` would mislead the backend
+ * about its origin, and `content-length` is recomputed by the upstream
+ * fetch from the body stream.
+ */
+const HOP_BY_HOP_HEADERS = [
+    'host',
+    'content-length',
+    'connection',
+    'keep-alive',
+    'proxy-authenticate',
+    'proxy-authorization',
+    'te',
+    'trailer',
+    'transfer-encoding',
+    'upgrade'
+];
+
+/**
+ * Cap each proxied request at 5 minutes. The bound exists to surface
+ * a wedged backend (stuck on a slow DB query, deadlocked, etc.) as a
+ * 502 rather than letting the browser request hang indefinitely.
+ *
+ * The default leans toward the slow-upload end of the spectrum: at a
+ * 1 Mbps upstream, a 200 MiB chapter upload (the default
+ * `MAX_REQUEST_BYTES` cap) needs ~27 minutes; 300 s covers the more
+ * realistic 25 Mbps urban-broadband case (~64 s for the same upload)
+ * with comfortable headroom. Operators serving very slow clients
+ * should raise `BACKEND_PROXY_TIMEOUT_MS`; operators behind a
+ * tighter upstream proxy may want to lower it. A future improvement
+ * is an idle-based timeout (reset per chunk) instead of this
+ * wall-clock budget — that's a fair bit more code, deferred.
+ */
+const PROXY_TIMEOUT_MS = (() => {
+    const raw = process.env.BACKEND_PROXY_TIMEOUT_MS;
+    const n = raw ? Number(raw) : 300_000;
+    return Number.isFinite(n) && n > 0 ? n : 300_000;
+})();
+
 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');
+        for (const h of HOP_BY_HOP_HEADERS) headers.delete(h);
-        headers.delete('content-length');
+
+        // AbortController times the upstream fetch out so a backend
+        // wedged on a slow DB query doesn't keep the browser request
+        // hanging forever. The `signal` is also wired into the
+        // RequestInit so the body stream is cancelled cleanly.
+        const ctrl = new AbortController();
+        const timeoutHandle = setTimeout(() => ctrl.abort(), PROXY_TIMEOUT_MS);
 
         const init: RequestInit & { duplex?: 'half' } = {
             method: event.request.method,
             headers,
-            redirect: 'manual'
+            redirect: 'manual',
+            signal: ctrl.signal
         };
         if (event.request.method !== 'GET' && event.request.method !== 'HEAD') {
             init.body = event.request.body;
@@ -39,11 +85,13 @@ export const handle: Handle = async ({ event, resolve }) => {
             upstream = await fetch(target, init);
         } catch (e) {
             // Network-layer failure (DNS / connection refused / TLS
-            // handshake) — most commonly "backend container restarting".
+            // handshake / abort by timeout) — most commonly "backend
-            // SvelteKit's default 500 would be an HTML page that
+            // container restarting". SvelteKit's default 500 would be
-            // client.ts can't .json(), which masks the real cause. Emit
+            // an HTML page that client.ts can't .json(), which masks
-            // the standard envelope with a dedicated code instead.
+            // the real cause. Emit the standard envelope with a
+            // dedicated code instead.
             console.error('Proxy to backend failed:', e);
+            clearTimeout(timeoutHandle);
             return new Response(
                 JSON.stringify({
                     error: {
@@ -58,6 +106,7 @@ export const handle: Handle = async ({ event, resolve }) => {
             );
         }
 
+        clearTimeout(timeoutHandle);
         return new Response(upstream.body, {
             status: upstream.status,
             statusText: upstream.statusText,