Friends system for app (#2958)

* Friends system for app

* Fix impl issues

* move friends to in-memory store

apps/labrinth/migrations/20241121232522_friends.sql

@@ -0,0 +1,11 @@
+CREATE TABLE friends (
+    user_id BIGINT NOT NULL REFERENCES users(id),
+    friend_id BIGINT NOT NULL REFERENCES users(id),
+    created TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    accepted BOOLEAN NOT NULL DEFAULT FALSE,
+
+    PRIMARY KEY (user_id, friend_id)
+);
+
+ALTER TABLE users
+    ADD COLUMN allow_friend_requests BOOLEAN NOT NULL DEFAULT TRUE;