fix: audit 89 bugs - critical fixes

CRITICAL:
- JwtMiddleware orden (antes de UseAuthentication)
- JWT Secret en appsettings.json (no vacio)
- Transbank MySQL connection string (User=root + placeholder)
- package-lock generado + Dockerfile usa npm install
- Tests E2E: selectores corregidos con name attributes
- Tests: rut invalido reemplazado, casos sin auth
- Contacto page: apunta a services-externos (no backend)
- .env.example: credenciales reales -> placeholders

HIGH:
- extra_hosts agregado a services-externos en compose
- Dockerfile frontend: npm ci -> npm install

services-externos/src/ServicesExternos.API/appsettings.json

@@ -8,7 +8,7 @@
   "AllowedHosts": "*",
   "ConnectionStrings": {
     "Default": "Host=192.168.0.254;Port=5432;Database=ichn;Username=postgres;Password=apoca11",
-    "CajaTbk": "Server=192.168.0.254;Port=3306;Database=caja_tbk;User=postgres;Password=apoca11"
+    "CajaTbk": "Server=192.168.0.254;Port=3306;Database=caja_tbk;User=root;Password=${MYSQL_CAJA_PASSWORD}"
   },
   "LibreDTE": {
     "UserHash": "",