fix: update existing cmd on conflict

When adding a new command, and the command in fact already exists, thus,
it's an update, replace the content of the command instead of failing.

queries/custom_cmds/add.sql

@@ -1 +1,2 @@
-INSERT INTO custom_commands (source, name, content) VALUES (?, ?, ?);
+INSERT INTO custom_commands (source, name, content) VALUES (?, ?, ?)
+ON CONFLICT (source, name) DO UPDATE SET content = excluded.content;

src/state.rs

@@ -223,4 +223,19 @@ mod tests {
 
         assert!(state.list_custom_commands().unwrap().is_empty());
     }
+
+    #[test]
+    fn overwrite_command() {
+        let state = State::in_memory().unwrap();
+
+        state
+            .add_custom_command(Source::Discord, "test", "one")
+            .unwrap();
+        state
+            .add_custom_command(Source::Discord, "test", "two")
+            .unwrap();
+
+        let cmd = state.get_custom_command(Source::Discord, "test").unwrap();
+        assert_eq!(Some("two"), cmd.as_deref());
+    }
 }