chore: clean up repo state and improve dev tooling

- Add IF NOT EXISTS to all migration DDL for idempotency
- Support PORT env var in backend (for compose flexibility)
- Support HOST_PORT env var in docker-compose
- Improve seed-demo to apply migrations before seeding
- Gitignore .claude/ session cache

docker-compose.yml

@@ -2,11 +2,12 @@ services:
   app:
     build: .
     ports:
-      - "3000:3000"
+      - "${HOST_PORT:-3000}:3000"
     volumes:
       - ./data:/data
     environment:
       - DATABASE_URL=sqlite:/data/attendance.db
       - STATIC_DIR=/app/frontend/build
       - JWT_SECRET=${JWT_SECRET:-dev_secret_for_demo}
+      - PORT=3000
     restart: always