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/app/api/v1/ai/scale/route.ts

@@ -4,7 +4,7 @@ import { db, recipes, eq, and, or } from "@epicure/db";
 import { requireSession } from "@/lib/api-auth";
 import { applyRateLimit } from "@/lib/rate-limit";
 import { getDefaultProviderWithKey } from "@/lib/ai/resolve-user-key";
-import { checkAndIncrementTierLimit } from "@/lib/tiers";
+import { withAiQuota } from "@/lib/ai/ai-error";
 import { scaleRecipe } from "@/lib/ai/features/scale-recipe";
 
 const Schema = z.object({
@@ -40,21 +40,22 @@ export async function POST(req: NextRequest) {
     return NextResponse.json({ error: "Not found" }, { status: 404 });
   }
 
-  await checkAndIncrementTierLimit(session!.user.id, session!.user.tier as "free" | "pro", "aiCall");
-
   const aiConfig = await getDefaultProviderWithKey(session!.user.id);
 
-  const scaledIngredients = await scaleRecipe(
-    {
-      title: recipe.title,
-      baseServings: recipe.baseServings,
-      ingredients: recipe.ingredients,
-    },
-    targetServings,
-    recipe.baseServings,
-    aiConfig,
-    (session!.user as { locale?: string }).locale ?? "en"
+  const result = await withAiQuota(session!.user.id, session!.user.tier as "free" | "pro", () =>
+    scaleRecipe(
+      {
+        title: recipe.title,
+        baseServings: recipe.baseServings,
+        ingredients: recipe.ingredients,
+      },
+      targetServings,
+      recipe.baseServings,
+      aiConfig,
+      (session!.user as { locale?: string }).locale ?? "en"
+    )
   );
+  if (!result.ok) return result.response;
 
-  return NextResponse.json({ ingredients: scaledIngredients });
+  return NextResponse.json({ ingredients: result.data });
 }