fix(trips): reject on-behalf trip creation without trips.manage

POST /trips honored body.user_id for any trips.record holder — impersonation
on create (reads were already scoped). Now an explicit Czech 403 unless the
caller has the manager capability; manager on-behalf flow unchanged. +4 tests.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/__tests__/trips.test.ts

@@ -19,6 +19,9 @@ import tripsRoutes from "../routes/admin/trips";
 //    WHOLE filtered set (not one 25-row page), honors the month filter in
 //    both wire formats ("month=5&year=2098" and "month=2098-05"), and scopes
 //    non-managers to their own trips exactly like the list does.
+// 3. POST /trips rejects a non-manager filing a trip under someone else's
+//    user_id with an explicit Czech 403 (impersonation gap), while
+//    self-creates and manager on-behalf creates keep working.
 // ---------------------------------------------------------------------------
 
 /** Note-prefix for test-created rows so cleanup never touches real data. */
@@ -66,6 +69,18 @@ async function authGet(path: string, token: string) {
   });
 }
 
+async function authPost(path: string, token: string, body: unknown) {
+  return app.inject({
+    method: "POST",
+    url: path,
+    headers: {
+      Authorization: `Bearer ${token}`,
+      "Content-Type": "application/json",
+    },
+    payload: body as object,
+  });
+}
+
 async function authPut(path: string, token: string, body: unknown) {
   return app.inject({
     method: "PUT",
@@ -539,3 +554,88 @@ describe("GET /api/admin/trips/stats", () => {
     expect(filtered.json().data.count).toBe(2);
   });
 });
+
+describe("POST /api/admin/trips — on-behalf authorization", () => {
+  function postBody(params: { userId?: number; note: string }) {
+    return {
+      vehicle_id: vehicleAId,
+      ...(params.userId !== undefined ? { user_id: params.userId } : {}),
+      trip_date: "2098-11-10",
+      start_km: 100,
+      end_km: 150,
+      route_from: "Praha",
+      route_to: "Brno",
+      is_business: true,
+      notes: `${N}${params.note}`,
+    };
+  }
+
+  it("rejects a non-manager filing a trip under another user's id with 403", async () => {
+    const res = await authPost(
+      "/api/admin/trips",
+      scopeToken,
+      postBody({ userId: adminUserId, note: "authz_spoof" }),
+    );
+
+    expect(res.statusCode).toBe(403);
+    expect(res.json()).toEqual({
+      success: false,
+      error: "Nemáte oprávnění zadat jízdu za jiného uživatele",
+    });
+
+    // …and the trip must NOT have been created.
+    const created = await prisma.trips.findFirst({
+      where: { notes: `${N}authz_spoof` },
+    });
+    expect(created).toBeNull();
+  });
+
+  it("lets a non-manager create a trip without user_id (defaults to self)", async () => {
+    const res = await authPost(
+      "/api/admin/trips",
+      scopeToken,
+      postBody({ note: "authz_self_implicit" }),
+    );
+
+    expect(res.statusCode).toBe(201);
+    expect(res.json().success).toBe(true);
+
+    const created = await prisma.trips.findFirst({
+      where: { notes: `${N}authz_self_implicit` },
+    });
+    expect(created).not.toBeNull();
+    expect(created!.user_id).toBe(scopeUserId);
+  });
+
+  it("lets a non-manager create a trip with their OWN user_id", async () => {
+    const res = await authPost(
+      "/api/admin/trips",
+      scopeToken,
+      postBody({ userId: scopeUserId, note: "authz_self_explicit" }),
+    );
+
+    expect(res.statusCode).toBe(201);
+
+    const created = await prisma.trips.findFirst({
+      where: { notes: `${N}authz_self_explicit` },
+    });
+    expect(created).not.toBeNull();
+    expect(created!.user_id).toBe(scopeUserId);
+  });
+
+  it("lets a manager/admin create a trip on behalf of another user", async () => {
+    const res = await authPost(
+      "/api/admin/trips",
+      adminToken,
+      postBody({ userId: scopeUserId, note: "authz_on_behalf" }),
+    );
+
+    expect(res.statusCode).toBe(201);
+
+    const created = await prisma.trips.findFirst({
+      where: { notes: `${N}authz_on_behalf` },
+    });
+    expect(created).not.toBeNull();
+    expect(created!.user_id).toBe(scopeUserId);
+  });
+});