feat: clear chat history manually, auto-expire old history after 90 days

Manual: DELETE /api/v1/ai/chat-history (scoped by recipeId or scope=general,
matching GET's scoping) plus a trash-icon button + confirm dialog in both
chat panels' headers.

Automatic: new internal cron endpoint (chat-cleanup), same shared-secret
pattern as the existing leftover-reminders/weekly-digest crons, deletes
any chat_messages older than 90 days. Wired into cron/crontab (daily,
03:00 UTC) and the Dockerfile's cron stage.

Verified locally: cleared a real conversation through the actual UI and
confirmed it didn't come back on reopen (not just cleared client-side);
inserted a 100-day-old and a 5-day-old message directly, called the cron
endpoint with the real shared-secret check, confirmed only the old one
was deleted and the recent one survived; confirmed the endpoint 401s with
no/wrong secret.

apps/web/app/api/v1/ai/chat-history/route.ts

@@ -51,3 +51,33 @@ export async function GET(req: NextRequest) {
     })),
   });
 }
+
+const DeleteSchema = z.object({
+  recipeId: z.string().uuid().optional(),
+  scope: z.enum(["general"]).optional(),
+});
+
+// No `q` here on purpose — clearing is "this whole conversation" (or
+// everything), not "every message matching a search term".
+export async function DELETE(req: NextRequest) {
+  const { session, response } = await requireSession();
+  if (response) return response;
+
+  const { searchParams } = new URL(req.url);
+  const parsed = DeleteSchema.safeParse({
+    recipeId: searchParams.get("recipeId") ?? undefined,
+    scope: searchParams.get("scope") ?? undefined,
+  });
+  if (!parsed.success) {
+    return NextResponse.json({ error: "Validation error" }, { status: 400 });
+  }
+  const { recipeId, scope } = parsed.data;
+
+  const conditions = [eq(chatMessages.userId, session!.user.id)];
+  if (recipeId) conditions.push(eq(chatMessages.recipeId, recipeId));
+  else if (scope === "general") conditions.push(isNull(chatMessages.recipeId));
+
+  await db.delete(chatMessages).where(and(...conditions));
+
+  return NextResponse.json({ ok: true });
+}