feat: add Prometheus metrics + fix CI/CD health check port

- Add prometheus-client dependency
- Create middleware/metrics.py with PrometheusMiddleware
- Expose /metrics endpoint in Prometheus text format
- Track http_requests_total, translation_total, translation_duration_seconds,
  file_size_bytes
- Instrument translate routes with record_translation() and record_file_size()
- Fix deploy.yml health check: localhost:8000 -> localhost:8001 (Portainer conflict)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.gitea/workflows/deploy.yml

@@ -52,7 +52,7 @@ jobs:
 
           # Final health check on backend
           for i in $(seq 1 10); do
-            if curl -sf http://localhost:8000/health > /dev/null 2>&1; then
+            if curl -sf http://localhost:8001/health > /dev/null 2>&1; then
               echo "Backend health check: OK"
               break
             fi
@@ -77,7 +77,7 @@ jobs:
           echo "========================================="
           docker compose ps
           echo ""
-          echo "Health: $(curl -sf http://localhost:8000/health 2>/dev/null || echo 'FAILED')"
+          echo "Health: $(curl -sf http://localhost:8001/health 2>/dev/null || echo 'FAILED')"
           echo "========================================="
 
   # Optional: deploy monitoring stack