Dedupe RSVPs and add tests for auto-RSVP on claim

Add migration to clean up duplicate RSVPs (keep earliest per event+name).
Add GetRsvpByName and DeleteDuplicateRsvps queries. Tests cover auto-RSVP
creation, duplicate prevention, and case-insensitive matching.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

db/queries.sql

@@ -72,6 +72,11 @@ SELECT COUNT(*) FROM rsvps WHERE event_id = ?;
 -- name: GetRsvpByName :one
 SELECT * FROM rsvps WHERE event_id = ? AND name = ? COLLATE NOCASE LIMIT 1;
 
+-- name: DeleteDuplicateRsvps :exec
+DELETE FROM rsvps WHERE id NOT IN (
+  SELECT MIN(id) FROM rsvps GROUP BY event_id, name COLLATE NOCASE
+);
+
 -- name: GetUserByPhone :one
 SELECT * FROM users WHERE phone = ?;