fix: improved discord & signal comms

migrations/20260205_discord_username.sql

@@ -0,0 +1,3 @@
+ALTER TABLE users ADD COLUMN discord_username TEXT;
+
+UPDATE users SET discord_id = NULL, discord_verified = FALSE WHERE discord_id IS NOT NULL;

migrations/20260206_signal_username.sql

@@ -0,0 +1 @@
+ALTER TABLE users RENAME COLUMN signal_number TO signal_username;