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/generate-from-idea/route.ts

@@ -2,7 +2,7 @@ import { NextRequest, NextResponse } from "next/server";
 import { z } from "zod";
 import { requireSession } from "@/lib/api-auth";
 import { applyRateLimit } from "@/lib/rate-limit";
-import { checkAndIncrementTierLimit } from "@/lib/tiers";
+import { withAiQuota } from "@/lib/ai/ai-error";
 import { generateRecipe } from "@/lib/ai/features/generate-recipe";
 import { getUserPrivateBio } from "@/lib/ai/user-bio";
 import { db, recipes, recipeIngredients, recipeSteps } from "@epicure/db";
@@ -29,17 +29,19 @@ export async function POST(req: NextRequest) {
   const limited = await applyRateLimit(`rl:ai:${session!.user.id}`, 10, 60);
   if (limited) return limited;
 
-  await checkAndIncrementTierLimit(session!.user.id, session!.user.tier as "free" | "pro", "aiCall");
-
   const privateBio = await getUserPrivateBio(session!.user.id);
   const locale = (session!.user as { locale?: string }).locale ?? "en";
 
-  const recipe = await generateRecipe(parsed.data.title, {
-    provider: parsed.data.provider,
-    model: parsed.data.model,
-    userContext: privateBio ?? undefined,
-    language: LANG[locale] ?? "English",
-  });
+  const result = await withAiQuota(session!.user.id, session!.user.tier as "free" | "pro", () =>
+    generateRecipe(parsed.data.title, {
+      provider: parsed.data.provider,
+      model: parsed.data.model,
+      userContext: privateBio ?? undefined,
+      language: LANG[locale] ?? "English",
+    })
+  );
+  if (!result.ok) return result.response;
+  const recipe = result.data;
 
   const recipeId = crypto.randomUUID();