Refactor GoogleService: streamline token handling and improve comments

src/google/google.service.ts

@@ -1,6 +1,5 @@
 import { BadRequestException, Injectable, Logger } from "@nestjs/common";
 import { google } from "googleapis";
-import { Credentials } from "google-auth-library";
 import { PrismaService } from "../prisma/prisma.service";
 
 const SCOPES = [
@@ -31,19 +30,18 @@ export class GoogleService {
     const authUrl = client.generateAuthUrl({
       access_type: "offline",
       scope: SCOPES,
-      prompt: "consent",
-      state: userId,
+      prompt: "consent", // always request a refresh_token
+      state: userId,     // passed back in callback to identify the user
     });
     return { authUrl };
   }
 
   async exchangeCode(userId: string, code: string) {
     const client = this.createClient();
-    let tokens: Credentials;
-
+    let tokens: import("google-auth-library").Credentials;
     try {
-      const { tokens: t } = await client.getToken(code);
-      tokens = t;
+      const result = await client.getToken(code);
+      tokens = result.tokens;
     } catch {
       throw new BadRequestException("Invalid or expired authorization code.");
     }
@@ -54,6 +52,7 @@ export class GoogleService {
       );
     }
 
+    // Fetch the Google account email
     client.setCredentials(tokens);
     const oauth2 = google.oauth2({ version: "v2", auth: client });
     const { data } = await oauth2.userinfo.get();
@@ -67,7 +66,7 @@ export class GoogleService {
         accessToken: tokens.access_token ?? null,
         isConnected: true,
         connectedAt: new Date(),
-        spreadsheetId: null,
+        spreadsheetId: null, // reset so a new spreadsheet is created on next export
       },
       create: {
         userId,