fix(discord-gateway): generate Drizzle migrations and copy to Docker image

Container crashed with 'Can't find meta/_journal.json file' because
Drizzle ORM migration files were never generated. Generated migrations
for all 10 schema tables and updated Dockerfile to copy drizzle/ to
/app/drizzle/ where the migrator expects them.

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

services/discord-gateway/drizzle.config.ts

@@ -0,0 +1,10 @@
+import { defineConfig } from "drizzle-kit";
+
+export default defineConfig({
+  schema: "./src/shared/database/schema.ts",
+  out: "./drizzle/migrations",
+  dialect: "postgresql",
+  dbCredentials: {
+    url: process.env.DATABASE_URL || "postgresql://postgres:postgres@localhost:5432/bete",
+  },
+});