fix(ai): forget the remembered working provider when the chain is saved

The in-memory memo of the last provider that answered reorders the
chain for up to an hour. Saving a new chain now drops it for that
account, so a provider the person removed or disabled no longer leads
the chain they just arranged.

src/app/api/insights/provider-chain/__tests__/put.test.ts

@@ -61,8 +61,14 @@ vi.mock("@/lib/logging/context", () => ({
   annotate: vi.fn(),
 }));
 
+vi.mock("@/lib/ai/provider-runner", () => ({
+  getLastWorkingProvider: vi.fn(),
+  forgetLastWorkingProvider: vi.fn(),
+}));
+
 import { PUT } from "../route";
 import { prisma } from "@/lib/db";
+import { forgetLastWorkingProvider } from "@/lib/ai/provider-runner";
 import { annotate } from "@/lib/logging/context";
 
 interface Envelope {
@@ -108,6 +114,9 @@ describe("PUT /api/insights/provider-chain", () => {
       ],
     });
     expect(annotate).toHaveBeenCalled();
+    // The remembered working provider would reorder the chain the person
+    // just arranged; a save drops it for this account.
+    expect(forgetLastWorkingProvider).toHaveBeenCalledWith("u-1");
   });
 
   it("rejects unknown provider types with 422", async () => {

src/app/api/insights/provider-chain/__tests__/route.test.ts

@@ -30,6 +30,7 @@ vi.mock("@/lib/ai/provider", () => ({
 
 vi.mock("@/lib/ai/provider-runner", () => ({
   getLastWorkingProvider: vi.fn(),
+  forgetLastWorkingProvider: vi.fn(),
 }));
 
 vi.mock("@/lib/logging/context", () => ({

src/app/api/insights/provider-chain/route.ts

@@ -10,7 +10,10 @@ import {
 } from "@/lib/api-response";
 import { annotate } from "@/lib/logging/context";
 import { resolveProviderChain } from "@/lib/ai/provider";
-import { getLastWorkingProvider } from "@/lib/ai/provider-runner";
+import {
+  forgetLastWorkingProvider,
+  getLastWorkingProvider,
+} from "@/lib/ai/provider-runner";
 import { prisma } from "@/lib/db";
 import {
   parseProviderChain,
@@ -149,6 +152,7 @@ export const PUT = apiHandler(async (request: NextRequest) => {
     where: { id: user.id },
     data: { aiProviderChain: normalised },
   });
+  forgetLastWorkingProvider(user.id);
 
   annotate({
     meta: {