refactor: break monorepo into 3 standalone services (gateway, backend, frontend)

- Remove pnpm workspace, moon repo, and all monorepo tooling
- Delete packages/shared/, embed shared code directly into each service
- Copy packages/shared/src/* -> services/backend/src/shared/ and services/discord-gateway/src/shared/
- Replace all @bete/shared imports with @/shared/ path alias
- Remove @bete/shared workspace dependency from both services
- Update root package.json scripts from --filter to --prefix
- Rewrite Dockerfiles to build each service standalone
- Clean up biome.json, .gitignore, remove root drizzle.config.ts

services/backend/package.json

@@ -14,7 +14,6 @@
     "test": "vitest run"
   },
   "dependencies": {
-    "@bete/shared": "workspace:*",
     "@discordjs/voice": "^0.19.2",
     "axios": "^1.16.1",
     "dotenv": "^17.4.2",