feat(chat): agregar selector de modelo de lenguaje en la interfaz

feat(ai): integrar Gemini y refactorizar AI Service con patron Factory

Update README.md

feat(backend): agregar nueva migración y actualizar vistas

docs(project): actualizar README y configuración Docker

feat(frontend): integracion total de dashboard, chat IA y configuraciones Docker (sin submódulo)

feat(chat): restauracion de modelos de chat y openai service, endpoints de mensajeria

merge: conservo frontend propio y backend remoto

chore: initial project setup with backend (Django + PostgreSQL)

chore: add missing subscription migration, update gitignore

test: add subscription and chats tests, 29/29 passing

fix(chats): add IsAuthenticated permission_classes to all views

fix(documents): add FileType.from_extension() classmethod

fix(users): remove duplicate imports, fix SubscriptionDetailView fields

feat(subscription): restore upgrade() and cancel(), add ready() for signals