add required UNIQUE constraints

Author: KavikaPalletenne

backend/migrations/20240406024211_create_organisations.sql

@@ -8,7 +8,7 @@ CREATE TABLE organisations (
 );
 
 CREATE TYPE organisation_role AS ENUM ('User', 'Admin');
-CREATE INDEX IDX_organisations_slug on organisations(slug);
+CREATE UNIQUE INDEX IDX_organisations_slug on organisations(slug);
 
 CREATE TABLE organisation_members (
     id BIGSERIAL PRIMARY KEY,

backend/migrations/20240406031915_create_applications.sql

@@ -18,7 +18,8 @@ CREATE TABLE applications (
         FOREIGN KEY(user_id)
             REFERENCES users(id)
             ON DELETE CASCADE
-            ON UPDATE CASCADE
+            ON UPDATE CASCADE,
+    UNIQUE(user_id, campaign_id)
 );
 
 CREATE INDEX IDX_applications_campaign on applications(campaign_id);