Fix possibility of extra mention insertion on nonce collision

crates/collab/migrations/20231018102700_create_mentions.sql

@@ -5,3 +5,7 @@ CREATE TABLE "channel_message_mentions" (
     "user_id" INTEGER NOT NULL REFERENCES users (id) ON DELETE CASCADE,
     PRIMARY KEY(message_id, start_offset)
 );
+
+-- We use 'on conflict update' with this index, so it should be per-user.
+CREATE UNIQUE INDEX "index_channel_messages_on_sender_id_nonce" ON "channel_messages" ("sender_id", "nonce");
+DROP INDEX "index_channel_messages_on_nonce";