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

.env.example

@@ -1,15 +1,18 @@
-# JWT
-JWT_SECRET=generar-clave-segura-aqui
+# JWT - CAMBIAR en produccion (min 32 caracteres)
+JWT_SECRET=cambiar-por-clave-segura-min-32-caracteres!!!
 JWT_EXPIRATION=30
 
 # LibreDTE
-LIBREDTE_USER_HASH=ZDLimhVCDEXoHR6yDTJpb80ta7KG4DqI
-LIBREDTE_AMBIENTE=0
+LIBREDTE_USER_HASH=cambiar-por-user-hash-real
+LIBREDTE_AMBIENTE=1
 
 # Transbank
-TRANSBANK_API_KEY=tu-api-key
-TRANSBANK_COMMERCE_CODE=tu-codigo-comercio
+TRANSBANK_API_KEY=cambiar-por-api-key
+TRANSBANK_COMMERCE_CODE=cambiar-por-commerce-code
 TRANSBANK_ENVIRONMENT=integration
 
+# MySQL caja_tbk (Transbank)
+MYSQL_CAJA_PASSWORD=cambiar-por-password
+
 # SMTP
-SMTP_PASSWORD=smith2251!
+SMTP_PASSWORD=cambiar-por-password