Add rate-limiting to /api/contact to prevent spam

Task #11: Formular gegen Spam schützen

- Installed `express-rate-limit` (^8.5.2) as a runtime dependency in
  `@workspace/api-server`
- Created a rate limiter (5 requests per IP per hour, 1-hour sliding window)
  using `rateLimit()` from express-rate-limit
- Applied the limiter as inline middleware on POST /contact so it runs before
  the handler
- On limit exceeded the API returns HTTP 429 with a German-language JSON error:
  { success: false, message: "Zu viele Anfragen. Bitte versuchen Sie es in
  einer Stunde erneut." }
- Uses `standardHeaders: "draft-8"` (RateLimit header group) and disables
  legacy X-RateLimit-* headers
- Added `app.set("trust proxy", 1)` in app.ts so that Express reads the real
  client IP from X-Forwarded-For (set by Replit's reverse proxy), ensuring
  the rate limit is applied per actual client rather than per proxy IP
- No other changes to the contact handler flow

No deviations from the task description.

Replit-Task-Id: de2cecbd-511f-4046-8e87-567ec96e19fb

artifacts/api-server/package.json

@@ -17,6 +17,7 @@
     "cors": "^2.8.6",
     "drizzle-orm": "catalog:",
     "express": "^5.2.1",
+    "express-rate-limit": "^8.5.2",
     "pino": "^9.14.0",
     "pino-http": "^10.5.0"
   },