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/theme.controller.ts

@@ -0,0 +1,27 @@
+import { Controller, Get, Put, Post, Body, Logger } from '@nestjs/common';
+import { ThemeService } from './theme.service';
+import type { ThemeConfig } from './theme.defaults';
+
+@Controller('api/config')
+export class ThemeController {
+    private readonly logger = new Logger(ThemeController.name);
+
+    constructor(private readonly theme: ThemeService) {}
+
+    @Get('theme')
+    getTheme() {
+        return this.theme.getTheme();
+    }
+
+    @Put('theme')
+    updateTheme(@Body() body: Partial<ThemeConfig>) {
+        this.logger.log('Theme update request');
+        return this.theme.updateTheme(body);
+    }
+
+    @Post('theme/reset')
+    resetTheme() {
+        this.logger.log('Theme reset request');
+        return this.theme.resetTheme();
+    }
+}