Basic unsubscribe endpoint

migrations/20250921113345_add_unsubscribe_token_to_subscriptions.sql

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+ALTER TABLE subscriptions ADD COLUMN unsubscribe_token TEXT UNIQUE;
+
+UPDATE subscriptions
+SET unsubscribe_token = left(md5(random()::text), 25) 
+WHERE status = 'confirmed' AND unsubscribe_token IS NULL;

migrations/20250921152839_add_constraint_to_subscription_tokens_subscriber_id_fkey.sql

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+ALTER TABLE subscription_tokens
+DROP CONSTRAINT subscription_tokens_subscriber_id_fkey;
+
+ALTER TABLE subscription_tokens
+ADD CONSTRAINT subscription_tokens_subscriber_id_fkey
+    FOREIGN KEY (subscriber_id)
+    REFERENCES subscriptions (id)
+    ON DELETE CASCADE;