feat: store timeouts in database and log them ending

we have to do this because discord doesn't notify us when a timeout
ends naturally, only when a moderator removes it early.

Catalogger.Backend/Database/Migrations/003_store_timeouts.up.sql

@@ -0,0 +1,9 @@
+create table timeouts (
+    id integer generated by default as identity primary key,
+    user_id bigint not null,
+    guild_id bigint not null,
+    moderator_id bigint,
+    until timestamptz not null
+);
+
+create unique index ix_timeouts_user_guild on timeouts (user_id, guild_id);