server: index on nzb_posts created_at, attempt to speed up homepage

server/migrations/20250127192339_nzb_posts_created_at_idx.down.sql

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+-- Add down migration script here
+DROP INDEX nzb_posts_created_at_idx;

server/migrations/20250127192339_nzb_posts_created_at_idx.up.sql

@@ -0,0 +1,2 @@
+-- Add up migration script here
+CREATE INDEX nzb_posts_created_at_idx ON nzb_posts USING btree (created_at);