fix: ensure reply_to column exists via ALTER TABLE in migration

internal/db/db.go

@@ -189,6 +189,9 @@ CREATE INDEX IF NOT EXISTS idx_webhooks_token ON webhooks(token);
 ALTER TABLE webhooks ADD COLUMN IF NOT EXISTS token_hash VARCHAR(64);
 CREATE INDEX IF NOT EXISTS idx_webhooks_token_hash ON webhooks(token_hash);
 
+-- Ensure reply_to column exists on messages (added after initial table creation)
+ALTER TABLE messages ADD COLUMN IF NOT EXISTS reply_to UUID REFERENCES messages(id) ON DELETE SET NULL;
+
 CREATE TABLE IF NOT EXISTS push_subscriptions (
 	id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
 	user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,