feat: multiple named conversations for the cooking assistant

The general assistant had exactly one conversation per user forever
(recipeId null on chat_messages) — no way to start fresh or organize
by topic. Adds an ai_conversations table (title, timestamps) and a
nullable conversationId FK on chat_messages; the assistant panel gets
a conversations menu to create/switch/rename/delete, auto-titling a
new conversation from its first question.

Per-recipe chat is untouched — each recipe already has one natural
thread, so multi-conversation support only applies to the homepage
assistant. Pre-existing general messages (no conversationId) aren't
migrated into the new model and won't appear in the conversation list.

Requires migration 0042 to run against a live DB — not applied in
this sandbox (no Docker here); run `pnpm db:migrate`.

v0.43.0

apps/web/app/api/v1/ai/conversations/route.ts

@@ -0,0 +1,33 @@
+import { NextRequest, NextResponse } from "next/server";
+import { db, aiConversations, eq, desc } from "@epicure/db";
+import { requireSessionOrApiKey } from "@/lib/api-auth";
+
+export async function GET(req: NextRequest) {
+  const { session, response } = await requireSessionOrApiKey(req);
+  if (response) return response;
+
+  const rows = await db.query.aiConversations.findMany({
+    where: eq(aiConversations.userId, session!.user.id),
+    orderBy: desc(aiConversations.updatedAt),
+  });
+
+  return NextResponse.json({
+    data: rows.map((r) => ({
+      id: r.id,
+      title: r.title,
+      createdAt: r.createdAt.toISOString(),
+      updatedAt: r.updatedAt.toISOString(),
+    })),
+  });
+}
+
+export async function POST(req: NextRequest) {
+  const { session, response } = await requireSessionOrApiKey(req);
+  if (response) return response;
+
+  const id = crypto.randomUUID();
+  const now = new Date();
+  await db.insert(aiConversations).values({ id, userId: session!.user.id, title: null, createdAt: now, updatedAt: now });
+
+  return NextResponse.json({ id, title: null, createdAt: now.toISOString(), updatedAt: now.toISOString() }, { status: 201 });
+}