fix: preserve cachedAt metadata on cache hits

apps/backend/src/modules/cache/cache.service.spec.ts

@@ -0,0 +1,65 @@
+import { ConfigService } from '@nestjs/config';
+import { CacheService } from './cache.service';
+
+describe('CacheService', () => {
+  const configService = {
+    get: vi.fn((_key: string, fallback?: unknown) => fallback),
+  } as unknown as ConfigService;
+
+  const createCache = () => ({
+    get: vi.fn(),
+    set: vi.fn(),
+  });
+
+  it('stores data with cachedAt metadata on cache miss', async () => {
+    vi.useFakeTimers();
+    vi.setSystemTime(new Date('2026-06-25T10:00:00.000Z'));
+    const cache = createCache();
+    cache.get.mockResolvedValue(undefined);
+    const service = new CacheService(cache as never, configService);
+
+    try {
+      const result = await service.getOrFetch('prefix', ['a'], async () => ({ value: 1 }), 'ttlKey');
+
+      expect(result).toEqual({
+        data: { value: 1 },
+        fromCache: false,
+        cachedAt: '2026-06-25T10:00:00.000Z',
+      });
+      expect(cache.set).toHaveBeenCalledWith(
+        'prefix:a',
+        { data: { value: 1 }, cachedAt: '2026-06-25T10:00:00.000Z' },
+        900,
+      );
+    } finally {
+      vi.useRealTimers();
+    }
+  });
+
+  it('returns cachedAt metadata on cache hit', async () => {
+    const cache = createCache();
+    cache.get.mockResolvedValue({
+      data: { value: 1 },
+      cachedAt: '2026-06-25T10:00:00.000Z',
+    });
+    const service = new CacheService(cache as never, configService);
+
+    const result = await service.getOrFetch('prefix', ['a'], async () => ({ value: 2 }), 'ttlKey');
+
+    expect(result).toEqual({
+      data: { value: 1 },
+      fromCache: true,
+      cachedAt: '2026-06-25T10:00:00.000Z',
+    });
+  });
+
+  it('supports legacy raw cache values during rollout', async () => {
+    const cache = createCache();
+    cache.get.mockResolvedValue({ value: 1 });
+    const service = new CacheService(cache as never, configService);
+
+    const result = await service.getOrFetch('prefix', ['a'], async () => ({ value: 2 }), 'ttlKey');
+
+    expect(result).toEqual({ data: { value: 1 }, fromCache: true, cachedAt: null });
+  });
+});

apps/backend/src/modules/cache/cache.service.ts

@@ -3,6 +3,11 @@ import { CACHE_MANAGER } from '@nestjs/cache-manager';
 import { Cache } from 'cache-manager';
 import { ConfigService } from '@nestjs/config';
 
+type CacheEntry<T> = {
+  data: T;
+  cachedAt: string;
+};
+
 @Injectable()
 export class CacheService {
   constructor(
@@ -18,6 +23,16 @@ export class CacheService {
     await this.cacheManager.set(key, value, ttl);
   }
 
+  private isCacheEntry<T>(value: unknown): value is CacheEntry<T> {
+    return (
+      typeof value === 'object' &&
+      value !== null &&
+      'data' in value &&
+      'cachedAt' in value &&
+      typeof (value as { cachedAt?: unknown }).cachedAt === 'string'
+    );
+  }
+
   private buildKey(...parts: string[]): string {
     return parts.join(':');
   }
@@ -31,14 +46,19 @@ export class CacheService {
     const key = this.buildKey(keyPrefix, ...keyParts);
     const ttl = this.configService.get<number>(`app.cache.${ttlConfigKey}`, 900);
 
-    const cached = await this.get<T>(key);
+    const cached = await this.get<CacheEntry<T> | T>(key);
     if (cached !== undefined) {
-      return { data: cached, fromCache: true, cachedAt: null };
+      if (this.isCacheEntry<T>(cached)) {
+        return { data: cached.data, fromCache: true, cachedAt: cached.cachedAt };
+      }
+
+      return { data: cached as T, fromCache: true, cachedAt: null };
     }
 
     const data = await fetchFn();
-    await this.set(key, data, ttl);
+    const cachedAt = new Date().toISOString();
+    await this.set(key, { data, cachedAt }, ttl);
 
-    return { data, fromCache: false, cachedAt: new Date().toISOString() };
+    return { data, fromCache: false, cachedAt };
   }
 }