Setup volunteerconnect database schema

[pythpythpython]

Make the Supabase schema script idempotent to allow safe re-running.

This PR adds DROP POLICY IF EXISTS and DROP TRIGGER IF EXISTS statements before creating RLS policies and triggers, resolving errors when the script is executed multiple times.

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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