Switch Discord notifications from webhook to user ID DMs

Uses the existing bot token to send DMs to users by their Discord user ID
instead of posting to a channel webhook.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

config/schema.sql

@@ -10,7 +10,7 @@ CREATE TABLE IF NOT EXISTS users (
 CREATE TABLE IF NOT EXISTS notifications (
     id UUID PRIMARY KEY,
     user_uuid UUID REFERENCES users(id) ON DELETE CASCADE UNIQUE,
-    discord_webhook VARCHAR(500),
+    discord_user_id VARCHAR(100),
     discord_enabled BOOLEAN DEFAULT FALSE,
     ntfy_topic VARCHAR(255),
     ntfy_enabled BOOLEAN DEFAULT FALSE,