Fix Postgres RLS policy syntax in launch readiness migration

[Rexxyin]

Motivation

• The migration used create policy if not exists, which Postgres flagged as a syntax error (42601) and caused the migration to fail.
• The intent is to make the migration idempotent and compatible with PostgreSQL so it can be re-run without parse errors.

Description

• Replaced each create policy if not exists with a drop policy if exists ...; create policy ... pair to maintain idempotence while avoiding unsupported syntax.
• Updated all three RLS policies on public.expense_settlements: Participant read settlements, Self manage settlement, and Self update settlement.
• Kept the original policy definitions and access logic unchanged while altering only the surrounding statements for compatibility.

Testing

• No automated SQL or migration tests were executed for this change because it is an SQL-only migration edit.
• The repository commit containing the change was created successfully with no test suite run.

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Codex Task