e2e: dispatch backup via source-group API

Per-host Run-backup is gone — the host_chrome partial still
renders the button but it's hard-disabled with a tooltip
pointing to per-source-group Run-now. The smoke test was
clicking that disabled button and waiting forever for a URL
change that would never happen.

Replace the navigation-based dispatch with two API calls:
create a source group covering the agent's /source mount,
then POST to /api/hosts/{id}/source-groups/{gid}/run. The
backup-status assertion at the end is unchanged — host record
is still the source of truth.

e2e/playwright/tests/lib/server.ts

@@ -107,6 +107,43 @@ export async function waitForHostStatus(
     throw new Error(`waitForHostStatus: timeout. Last seen: ${JSON.stringify(last)}`);
 }
 
+export async function createSourceGroup(
+    request: APIRequestContext,
+    cookie: string,
+    hostID: string,
+    body: { name: string; includes: string[]; excludes?: string[] },
+): Promise<string> {
+    const res = await request.post(`${baseURL}/api/hosts/${hostID}/source-groups`, {
+        headers: { cookie, 'content-type': 'application/json' },
+        data: {
+            name: body.name,
+            includes: body.includes,
+            excludes: body.excludes ?? [],
+            retention_policy: {},
+            retry_max: 0,
+            retry_backoff_seconds: 0,
+        },
+    });
+    if (!res.ok()) throw new Error(`createSourceGroup: ${res.status()} ${await res.text()}`);
+    const created = (await res.json()) as { id?: string; group?: { id?: string } };
+    const id = created.id ?? created.group?.id;
+    if (!id) throw new Error(`createSourceGroup: no id in response: ${JSON.stringify(created)}`);
+    return id;
+}
+
+export async function runSourceGroup(
+    request: APIRequestContext,
+    cookie: string,
+    hostID: string,
+    groupID: string,
+): Promise<void> {
+    const res = await request.post(
+        `${baseURL}/api/hosts/${hostID}/source-groups/${groupID}/run`,
+        { headers: { cookie } },
+    );
+    if (!res.ok()) throw new Error(`runSourceGroup: ${res.status()} ${await res.text()}`);
+}
+
 export async function getSessionCookie(page: Page): Promise<string> {
     const cookies = await page.context().cookies();
     const c = cookies.find((c) => c.name === 'rm_session');