Implement bulletproof rejection flow

- Adds atomic rejection transaction edge function and RPC
- Updates moderation client to use new rejection path
- Introduces rejection transaction migration and supporting readouts
- Moves photo-related approval handling toward RPC-based flow
- Lays groundwork for idempotency and resilience in moderation actions

supabase/functions/process-selective-rejection/cors.ts

@@ -0,0 +1,4 @@
+export const corsHeaders = {
+  'Access-Control-Allow-Origin': '*',
+  'Access-Control-Allow-Headers': 'authorization, x-client-info, apikey, content-type',
+};