Foreign Key Violation (23503) on Insert Despite Successful Pre-Check in Vercel Serverless Function

**Describe the bug** When attempting to insert a row into a table (slack_accounts) that has a foreign key relationship to the users table within a Vercel serverless function (Next.js API route), the insert fails with a 23503 Foreign Key Violation. This happens immediately after a successful SELECT query confirms that the referenced user_id does exist in the users table within the same function execution context. This suggests a potential data visibility or replication lag issue within the Supabase/Vercel environment. **To Reproduce** **Setup:** Next.js application deployed on Vercel. Supabase Postgres database. Authentication handled by NextAuth.js, using Google Provider. users table (stores basic user info, primary key id is UUID). slack_accounts table (stores Slack tokens, has a user_id column with a foreign key constraint referencing users(id)). Supabase JavaScript client library (@supabase/supabase-js version: [Specify version if known, e.g., ^2.x.x]). **Flow:** User signs in via Google using NextAuth (/api/auth/signin/google). The NextAuth signIn and session callbacks (lib/authOptions.ts) successfully find or create a user record in the Supabase users table and store the user's Supabase UUID (users.id) in the session JWT (session.user.id). User initiates Slack OAuth flow (/api/slack/oauth/start), which redirects to Slack. User authorizes Slack, Slack redirects back to the callback route (/api/slack/oauth/callback). The callback route retrieves the authenticated user's Supabase UUID (appUserId) from the session. (Issue Point): The callback route attempts to save the obtained Slack token: An explicit SELECT id FROM users WHERE id = $1 query is performed using appUserId. This query successfully finds the user record. (Confirmed via Vercel logs). Immediately following the successful SELECT, an INSERT (or UPSERT) operation is performed on the slack_accounts table, including the same appUserId in the user_id column. This INSERT/UPSERT fails with PostgreSQL error code…