diff --git a/src/auth/auth.service.captcha.spec.ts b/src/auth/auth.service.captcha.spec.ts
index 7943fa08..5833e3e8 100644
--- a/src/auth/auth.service.captcha.spec.ts
+++ b/src/auth/auth.service.captcha.spec.ts
@@ -171,6 +171,51 @@ describe('AuthService – CAPTCHA failure lockout', () => {
     });
   });
 
+  describe('5 CAPTCHA failures trigger account lockout', () => {
+    it('locks account after 5 invalid CAPTCHA submissions', async () => {
+      mockCaptchaFail();
+      rateLimitService.isAccountLocked
+        .mockResolvedValueOnce(false)
+        .mockResolvedValueOnce(false)
+        .mockResolvedValueOnce(false)
+        .mockResolvedValueOnce(false)
+        .mockResolvedValueOnce(false);
+
+      rateLimitService.getFailedAttemptsCount.mockResolvedValue(3);
+
+      let callCount = 0;
+      rateLimitService.recordFailedAttempt.mockImplementation(async () => {
+        callCount++;
+        // After 5th failed attempt, the account is considered locked
+        if (callCount >= 5) {
+          rateLimitService.isAccountLocked.mockResolvedValue(true);
+          rateLimitService.getLockoutInfo.mockResolvedValue({
+            isLocked: true,
+            remainingLockoutMinutes: 30,
+          });
+          return true;
+        }
+        return false;
+      });
+
+      const loginArgs = {
+        email: 'user@example.com',
+        password: 'pass',
+        captchaToken: 'bad',
+      };
+
+      // First 5 attempts fail with "Invalid CAPTCHA"
+      for (let i = 0; i < 5; i++) {
+        await expect(service.login(loginArgs)).rejects.toThrow('Invalid CAPTCHA');
+      }
+
+      // 6th attempt: account is locked
+      await expect(service.login(loginArgs)).rejects.toThrow(/locked/i);
+
+      expect(rateLimitService.recordFailedAttempt).toHaveBeenCalledTimes(5);
+    });
+  });
+
   describe('password failure (unchanged behavior)', () => {
     it('still records a failed attempt on wrong password', async () => {
       rateLimitService.isAccountLocked.mockResolvedValue(false);
diff --git a/src/cache/cache-metrics.interceptor.ts b/src/cache/cache-metrics.interceptor.ts
index b946d0ce..e120857f 100644
--- a/src/cache/cache-metrics.interceptor.ts
+++ b/src/cache/cache-metrics.interceptor.ts
@@ -3,6 +3,24 @@
 /**
  * Cache Metrics Interceptor
  * Automatically tracks cache performance metrics
+ *
+ * DI Contract:
+ * This interceptor must be registered via app.useGlobalInterceptors() in main.ts
+ * using the already-resolved singleton from the DI container:
+ *
+ *   const cacheMetricsInterceptor = app.get(CacheMetricsInterceptor);
+ *   app.useGlobalInterceptors(cacheMetricsInterceptor);
+ *
+ * WHY: The CacheMonitoringService it depends on is provided at the CacheModule
+ * level. If @UseInterceptors(CacheMetricsInterceptor) were used on a controller
+ * instead, NestJS would instantiate a NEW interceptor instance from a sub-module
+ * injector, which would receive a DIFFERENT CacheMonitoringService singleton.
+ * As a result, metric counters would not reflect server-wide values, the stats
+ * endpoint would be unreliable, and alerting (low hit-rate / high response-time)
+ * would break silently.
+ *
+ * Any refactoring that changes how this interceptor is registered MUST verify
+ * that CacheMonitoringService remains a true application-wide singleton.
  */
 
 import { Injectable, NestInterceptor, ExecutionContext, CallHandler } from '@nestjs/common';
diff --git a/test/cache/cache-metrics.e2e-spec.ts b/test/cache/cache-metrics.e2e-spec.ts
new file mode 100644
index 00000000..d1682320
--- /dev/null
+++ b/test/cache/cache-metrics.e2e-spec.ts
@@ -0,0 +1,78 @@
+import { INestApplication } from '@nestjs/common';
+import { Test } from '@nestjs/testing';
+import * as request from 'supertest';
+import { AppModule } from '../../src/app.module';
+import { PrismaService } from '../../src/database/prisma.service';
+import { CacheMonitoringService } from '../../src/cache/cache-monitoring.service';
+
+class FakePrismaService {
+  users = new Map<string, any>();
+  blacklistedToken = new Map<string, any>();
+
+  async $connect() {}
+  async $disconnect() {}
+
+  user = {
+    findUnique: async ({ where }: any) => {
+      if (where?.id) return this.users.get(where.id) ?? null;
+      if (where?.email) return Array.from(this.users.values()).find((u) => u.email === where.email) ?? null;
+      return null;
+    },
+    update: async ({ where, data }: any) => {
+      const user = this.users.get(where.id);
+      const updated = { ...user, ...data };
+      this.users.set(where.id, updated);
+      return updated;
+    },
+  } as any;
+}
+
+describe('CacheMetricsInterceptor e2e — singleton verification', () => {
+  let app: INestApplication;
+  let monitoringService: CacheMonitoringService;
+  let fakePrisma: FakePrismaService;
+
+  beforeAll(async () => {
+    fakePrisma = new FakePrismaService();
+
+    const moduleRef = await Test.createTestingModule({ imports: [AppModule] })
+      .overrideProvider(PrismaService)
+      .useValue(fakePrisma as any)
+      .compile();
+
+    app = moduleRef.createNestApplication();
+    await app.init();
+
+    monitoringService = app.get(CacheMonitoringService);
+    monitoringService.resetMetrics();
+  }, 20000);
+
+  afterAll(async () => {
+    await app.close();
+  });
+
+  it('records metrics after an HTTP request', async () => {
+    const metricsBefore = monitoringService.getMetrics();
+    expect(metricsBefore.totalRequests).toBe(0);
+
+    await request(app.getHttpServer())
+      .get('/api/properties')
+      .expect(200);
+
+    const metricsAfter = monitoringService.getMetrics();
+    expect(metricsAfter.totalRequests).toBeGreaterThanOrEqual(1);
+    expect(metricsAfter.avgResponseTime).toBeGreaterThan(0);
+  });
+
+  it('accumulates metrics across multiple requests', async () => {
+    monitoringService.resetMetrics();
+
+    await request(app.getHttpServer()).get('/api/properties');
+    await request(app.getHttpServer()).get('/api/properties');
+    await request(app.getHttpServer()).get('/api/properties');
+
+    const metrics = monitoringService.getMetrics();
+    expect(metrics.totalRequests).toBeGreaterThanOrEqual(3);
+    expect(metrics.avgResponseTime).toBeGreaterThan(0);
+  });
+});