Add unique index for topic and callback

db/01_tables.sql

@@ -1,4 +1,4 @@
-create table "subscribers"
+CREATE TABLE IF NOT EXISTS "subscribers"
 (
   id            int GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
   topic         varchar(255) not null,

db/02_unique.sql

@@ -0,0 +1 @@
+CREATE UNIQUE INDEX IF NOT EXISTS "subscribers_topic_callback" ON "subscribers" ("topic", "callback");