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>

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);
+    });
 });