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/chat-history/route.ts

@@ -5,6 +5,7 @@ import { requireSessionOrApiKey } from "@/lib/api-auth";
 
 const Schema = z.object({
   recipeId: z.string().uuid().optional(),
+  conversationId: z.string().uuid().optional(),
   // "general" restricts to the homepage cooking assistant (recipeId null);
   // omit both recipeId and scope to search across everything.
   scope: z.enum(["general"]).optional(),
@@ -19,6 +20,7 @@ export async function GET(req: NextRequest) {
   const { searchParams } = new URL(req.url);
   const parsed = Schema.safeParse({
     recipeId: searchParams.get("recipeId") ?? undefined,
+    conversationId: searchParams.get("conversationId") ?? undefined,
     scope: searchParams.get("scope") ?? undefined,
     q: searchParams.get("q") ?? undefined,
     limit: searchParams.get("limit") ?? undefined,
@@ -26,10 +28,11 @@ export async function GET(req: NextRequest) {
   if (!parsed.success) {
     return NextResponse.json({ error: "Validation error" }, { status: 400 });
   }
-  const { recipeId, scope, q, limit } = parsed.data;
+  const { recipeId, conversationId, scope, q, limit } = parsed.data;
 
   const conditions = [eq(chatMessages.userId, session!.user.id)];
   if (recipeId) conditions.push(eq(chatMessages.recipeId, recipeId));
+  else if (conversationId) conditions.push(eq(chatMessages.conversationId, conversationId));
   else if (scope === "general") conditions.push(isNull(chatMessages.recipeId));
   if (q?.trim()) conditions.push(ilike(chatMessages.content, `%${q.trim()}%`));