feat: theme config service — REST API with versioning + backup

- ThemeService: read/write/validate theme.json, auto-backup on save
- ThemeController: GET/PUT/POST /api/config/theme (public GET, versioned PUT)
- ThemeConfig type with version + updatedAt fields
- Default theme: Global Hospital blue scale
- ConfigThemeModule registered in AppModule

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/config/config-theme.module.ts

@@ -0,0 +1,10 @@
+import { Module } from '@nestjs/common';
+import { ThemeController } from './theme.controller';
+import { ThemeService } from './theme.service';
+
+@Module({
+    controllers: [ThemeController],
+    providers: [ThemeService],
+    exports: [ThemeService],
+})
+export class ConfigThemeModule {}