refactor: use database transaction RPC for local coding sync

[bhavyanjain3004]

Summary

Resolves #816. Refactors the local coding sessions sync endpoint to use a database-level transaction function (batch_upsert_sessions RPC). This ensures atomic writes and prevents PostgreSQL cardinality errors when duplicate date entries are present in a single batch.

Changes

• Database Migration & Schema: Added a new migration for the batch_upsert_sessions(sessions jsonb) function and updated schema.sql.
• API Refactoring: Replaced .upsert() with supabaseAdmin.rpc("batch_upsert_sessions") in src/app/api/local-coding/sync/route.ts.
• Unit Tests: Added 10 tests in test/local-coding-sync.test.ts to verify authentication, validations, database limits, and RPC success/error handling.

How to Verify

1. Running the Automated Tests

Run the newly added unit tests specifically written for this endpoint:

npm test test/local-coding-sync.test.ts

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