Add RLS write policies for rebalances table

[robertocarlous]

Description

The rebalances table has public read access but no write policy. Only the agent or service role should be able to insert rebalance records.

Current State

• Public SELECT policy exists
• No INSERT/UPDATE policy
• Agent cannot write rebalance records

Expected Behavior

• Agent role should have INSERT permission on rebalances table
• Service role should have full access for administrative operations

Files Affected

• supabase/migrations/20260728000000_init_neurowealth_schema.sql