fix(database): use public schema for drizzle migrations to avoid permission error

PostgreSQL error 42501 when Drizzle tries CREATE SCHEMA IF NOT EXISTS
"drizzle" — the DB user lacks schema creation privileges. Configuring
migrationsSchema: "public" keeps __drizzle_migrations in the existing
public schema, matching what seedDrizzleHistory already expects.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

services/discord-gateway/src/shared/database/migrate.ts

@@ -139,6 +139,7 @@ export async function runMigrations(): Promise<void> {
 
           await migratePostgres(db, {
             migrationsFolder: "./drizzle/migrations",
+            migrationsSchema: "public",
           });
         } finally {
           await client.query("SELECT pg_advisory_unlock($1, $2)", [