feat : initial PostgreSQL infrastructure setup

- docker-compose with postgres:16-alpine
- Init script to create databases (SIRH + Ferme, prod + recette)
- Deploy script with readiness check
- Backup script with rotation (keeps last 7)
- Auto-tag CI workflow
- Full deployment documentation

.env.example

@@ -0,0 +1,3 @@
+# Superuser PostgreSQL (utilise pour creer les BDD/users au premier lancement)
+POSTGRES_USER=admin
+POSTGRES_PASSWORD=change-me