feat: Add loot builder backend + static data + DB migration

- Migration 013: loot_profiles table (JSONB loot_table + loot_groups, license-scoped)
- TypeORM entity matching migration schema exactly
- NestJS loot module: 10 endpoints (CRUD, duplicate, apply, import, export, containers)
- Multiplier logic recursively scales Min/Max/Scrap across loot tables and groups
- Apply-to-server writes BetterLoot JSON via NATS file manager + RCON reload
- Frontend static data: 191 Rust items, 51 container prefabs
- TypeScript types for BetterLoot data model (PrefabLoot, LootEntry, LootRNG, etc.)
- Fix vue-tsc errors: UngroupedItems uses LootRNG, null safety in store/view

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

frontend/src/stores/loot.ts

@@ -38,7 +38,7 @@ export const useLootStore = defineStore('loot', () => {
       // Select first container if none selected
       if (!selectedContainer.value && currentProfile.value.loot_table) {
         const keys = Object.keys(currentProfile.value.loot_table)
-        if (keys.length > 0) selectedContainer.value = keys[0]
+        if (keys.length > 0) selectedContainer.value = keys[0] ?? null
       }
     } catch (err) {
       toast.error((err as Error).message)