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

docker-compose.yml

@@ -0,0 +1,11 @@
+services:
+  postgres:
+    image: postgres:16-alpine
+    container_name: postgres
+    env_file: .env
+    ports:
+      - "5432:5432"
+    volumes:
+      - ./data:/var/lib/postgresql/data
+      - ./init:/docker-entrypoint-initdb.d:ro
+    restart: unless-stopped