fix: return clean errors and refund quota on AI provider failures

AI routes had no consistent error handling — a provider failure (e.g.
OpenRouter returning a degraded-model error) crashed the route as an
uncaught 500 and, worse, still charged the user's monthly aiCall
quota for a request that never succeeded.

Adds withAiQuota()/aiErrorResponse() (lib/ai/ai-error.ts) and applies
them across all 12 AI generation routes: charges the quota, runs the
AI call, refunds the credit and returns a clean user-facing message
if it throws. Frontend needs no changes — existing dialogs already
toast whatever `error` string comes back.

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

apps/web/lib/__tests__/tiers.test.ts

@@ -23,7 +23,7 @@ vi.mock("@epicure/db", () => ({
 }));
 
 // Import after mock
-const { checkAndIncrementTierLimit, incrementUsage } = await import("../tiers");
+const { checkAndIncrementTierLimit, incrementUsage, refundAiCall } = await import("../tiers");
 
 function makeChain(finalValue: unknown) {
   const chain = {
@@ -93,6 +93,14 @@ describe("checkAndIncrementTierLimit", () => {
   });
 });
 
+describe("refundAiCall", () => {
+  it("issues a decrement-with-floor UPDATE for the current month", async () => {
+    mockDb.execute.mockResolvedValueOnce(undefined);
+    await refundAiCall("user1");
+    expect(mockDb.execute).toHaveBeenCalledTimes(1);
+  });
+});
+
 describe("incrementUsage", () => {
   it("calls insert with correct aiCall initial values", async () => {
     const chain = makeInsertChain();