Initial affiliate codes implementation (#4382)

* Initial affiliate codes implementation

* some more docs to codes

* sqlx prepare

* Address PR comments

* Address more PR comments

* fix clippy

* Switch to using Json<T> for type-safe responses

apps/labrinth/migrations/20250914190749_affiliate_codes.sql

@@ -0,0 +1,9 @@
+CREATE TABLE affiliate_codes (
+	id            bigint      PRIMARY KEY,
+	created_at    timestamptz NOT NULL DEFAULT CURRENT_TIMESTAMP,
+	created_by    bigint      NOT NULL REFERENCES users(id),
+	affiliate     bigint      NOT NULL REFERENCES users(id),
+	-- left nullable so we can explicitly set payouts if we need to,
+	-- and use a global default if unset
+	revenue_split float
+);