feat(security): address findings from v2 security assessment

- L2: Add server_tokens off to nginx configs to hide version
- M1: Add X-Frame-Options, X-Content-Type-Options, Referrer-Policy,
  Permissions-Policy headers to all nginx routes
- L3: Add global NoCacheInterceptor (Cache-Control: no-store) on all
  API responses to prevent caching of sensitive financial data
- C1: Disable open registration by default (ALLOW_OPEN_REGISTRATION env)
- H3: Add logout endpoint with correct HTTP 200 status code
- M2: Implement full password reset flow (forgot-password, reset-password,
  change-password) with hashed tokens, 15-min expiry, single-use
- Reduce JWT access token expiry from 24h to 1h
- Add EmailService stub (logs to shared.email_log)
- Add DB migration 016 for password_reset_tokens table

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

backend/src/common/interceptors/no-cache.interceptor.ts

@@ -0,0 +1,16 @@
+import { Injectable, NestInterceptor, ExecutionContext, CallHandler } from '@nestjs/common';
+import { Observable } from 'rxjs';
+
+/**
+ * Prevents browsers and proxies from caching authenticated API responses
+ * containing sensitive financial data (account balances, transactions, PII).
+ */
+@Injectable()
+export class NoCacheInterceptor implements NestInterceptor {
+  intercept(context: ExecutionContext, next: CallHandler): Observable<any> {
+    const res = context.switchToHttp().getResponse();
+    res.setHeader('Cache-Control', 'no-store, no-cache, must-revalidate, private');
+    res.setHeader('Pragma', 'no-cache');
+    return next.handle();
+  }
+}