security: add helmet, rate limiting, webhook auth, fix token storage, restrict hard deletes

- Add helmet for HTTP security headers (CSP, HSTS, X-Frame-Options, etc.)
- Add @nestjs/throttler for rate limiting (100 req/60s per IP)
- Add shared secret validation on Signal webhook endpoint
- Remove JWT token from localStorage, use Auth0 SDK memory cache
  with async getAccessTokenSilently() in API interceptor
- Restrict hard delete (?hard=true) to ADMINISTRATOR role in service layer
- Replace exposed Anthropic API key with placeholder in .env

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/src/vehicles/vehicles.service.ts

@@ -1,4 +1,4 @@
-import { Injectable, NotFoundException, Logger } from '@nestjs/common';
+import { Injectable, NotFoundException, ForbiddenException, Logger } from '@nestjs/common';
 import { PrismaService } from '../prisma/prisma.service';
 import { CreateVehicleDto, UpdateVehicleDto } from './dto';
 
@@ -89,7 +89,11 @@ export class VehiclesService {
     });
   }
 
-  async remove(id: string, hardDelete = false) {
+  async remove(id: string, hardDelete = false, userRole?: string) {
+    if (hardDelete && userRole !== 'ADMINISTRATOR') {
+      throw new ForbiddenException('Only administrators can permanently delete records');
+    }
+
     const vehicle = await this.findOne(id);
 
     if (hardDelete) {