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/.sqlx/query-b36457e1aa73b5517ca4f41e4144084a0aa996168d2660c13a1d4c9fc4594859.json

@@ -0,0 +1,22 @@
+{
+  "db_name": "PostgreSQL",
+  "query": "SELECT EXISTS(SELECT 1 FROM affiliate_codes WHERE id=$1)",
+  "describe": {
+    "columns": [
+      {
+        "ordinal": 0,
+        "name": "exists",
+        "type_info": "Bool"
+      }
+    ],
+    "parameters": {
+      "Left": [
+        "Int8"
+      ]
+    },
+    "nullable": [
+      null
+    ]
+  },
+  "hash": "b36457e1aa73b5517ca4f41e4144084a0aa996168d2660c13a1d4c9fc4594859"
+}